Sounds like a suction bypass situation. Have you checked if the dust bag is full ... or the collection big needs emptying if it's a bagless unit?

You have a clog somewhere. Some vacuum cleaners have a valve, which opens up to allow air to continue to flow, when airflow is cut off. This is the "hissing"sound you hear, coming out of a small valve opening. If there was no valve,but hopefully a thermal switch, the vacuum would shut off after getting overheated.

What you need to do is find out where there is a clog. The common place would be the hose or the inner bagless bin filter.

If your vacuum will run without the hose, try that and see if the problem goes away or stays.
If it goes away there is likely a partial hose blockage.
One way to get some unreachable stuff to move is dropping suitably sized sockets from a tool set down the removed hose.
They are quite heavy for their size and can knock most clogs back to the nozzle for extraction.

I have just tested this and determinded that the cause is not in hose per se but what I have found after is when I remove canister with filter that must be often cleaned from dust and dirt, the vacuum cleaner does not longer make hissing. I don't think how canister with filter can be the cause because I did not drop it and no dents can be observed.

Maybe the cause is the above compartment that supposedly lets the air out of filter leaving dust and dirt in it? I have attached picture to this post so you could see better what I am talking about.

Have you tried putting in a fresh clean filter?
Does the exhaust area have yet another filter that needs to be replaced? Some of my vacs have that extra exhaust filter, and I have to check it regularly.

I said in my original post, "...the inner bagless bin filter."

This filter must be replaced at some point, but depends on the brand. You can test and see if it is the filter by removing the filter from the dirt container, then reinstall the dirt bin, without the filter, in the vacuum. Turn the vacuum on and without sucking up anything, see if you still have the "hissing" sound. If you do not have the sound, it is your filter.

I do not know what your manual says, but most will tell you to rinse under water or replace after a certain amount of months.

I have inspected HEPA filter closely and noticed that it has small layers of seems to be hard to remove dirt as you can observe on picture below. I did wash filter after every 2 cleaning routines under hot tap water as according to instructions which mention that even soft cleaner can be used as well. Manual however says nothing about changing this filter and mentions that it supposed to be permanent filter although the filter that must be changed every year is hot air exhaust filter. If anyone has any idea on how to get rid of hardened dirt on HEPA filter and can suggest chemical solution that wouldn't damage it, I would appreciate your input.

Here is a link that might be useful:

It looks like you are having a hard time believing anything I say.

What is the brand, model name and model number of the vacuum you have?

What you should do after emptying the dirt bin is take your finger and rub it over every pleat of the filter. As you do it hard enough and quick enough, dirt will dislodge from the pleats, because of movement and vibration. Think of it another way, when we were boys(at least in the USA), we would attach cards to our wheel spokes of our bikes. As we rode, the card would flick against each spoke; making a clicking sound. You are virtually doing the same thing. As you rub your finger over each pleat, it moves and vibrates each pleat; dislodging dirt. Some Panasonic made bagless vacuum cleaners utilized this technique, where plastic "fingers" would rub over the pleats of the filter, with just a turn of a knob. If you are rubbing every pleat good enough, you should be making a small "clicking" noise and see dust fall out.

Another thing to do, in between filter rinses, is take the dusting brush attachment(if your model has this attachment) and brush in between the pleats. You could use something like a soft paint brush also. Be careful to not wear the filter material out. Some vacuum brands tell the user not to do this. Brands which prohibit are usually using a thin paper filter. Tapping the filter against the trash can will help dislodge thick layers of collected dirt from the filter. Just make sure the plastic filter frame is the only thing hitting against the trash can. This is usually done before flicking the filter pleats with your fingers.

Be sure to do all of this outdoors, because it can get messy.

Let me say again, I listened to your short video originally and knew what was going on. You partially did what I said, and noticed a difference, but you still do not believe. I said nothing about dropping any part of the vacuum. Most bagless vacuums have the filter in the dirt bin. This filter will usually come apart from the lid or base of the dirt bin. It will either unscrew or lift out of the bin lid or base. If you remove the filter, then replace the lid, then reinstall the bin in the vacuum, you can then turn the vacuum on and listen. Do not pick up any dirt, while doing this test. If I knew the brand and model number, I would know for sure how your filter is installs in the dirt bin.

@geguymw Thanks for your help.

Well that was a long and daunting task I would say! The advertisements make it easier to sound like "effortless vacuum cleaner" because no need to bother changing bags constantly, but had I known this bagless filter requires such cleaning, I would have considered different vacuum cleaner WITH bags.

Thanks for I have done as you've said and definitely HEPA filter was cogged with dense stripes of dirt as you can see on picture below this message. The dirt did not budge with common bursh so I had to use small flat screwdriver to carefully clean them out and then use brush to dispose of last small particles of dirt. I have accidently tore a few synthetic threads but nothing major and I think this filter is made of special paper.

I am not sure how dirt became hardened because I cleaned filter after every 2-3 uses of vacuum cleaner and next time washed it under hot tap water. Maybe I had to use specific filter solution if there is such a thing? Manual mentioned nothing about it and it stated that filter can be used after 24 hours after its cleaning which I did. I just want to find the reason it was so clogged with hard to remove dirt because I don't want to tinker with it again any time soon.

So, now after filter is clean, should I also run it under hot tap water and let it dry before my next use?

Wow, that is some impressive dirt you removed from the filter.

Manual mentioned nothing about it and it stated that filter can be used after 24 hours after its cleaning which I did.
My guess is that perhaps the filter was not completely dry, and that the dirt became mud when it landed on the filter and became damp.

You need to work harder at removing as much of the dirt, from the filter, after each vacuuming. There is no need to wash it every time, but you do need to do what I described above. Tap the filter against the trash bin, by the plastic frame. Flick the filter pleats with your finger. Every month or two, rinse the filter under the sink faucet. If you can remove as much dirt before you rinse, you will have less work rinsing.

You still have not told me what brand and model vacuum cleaner you have.

Bagged vacuum cleaners require less maintenance than bagless and bagless with those bin filter are the worst.

@geguym

Sorry about my remiss of your question regarding brand and model. It is UFESA AC5918 which you can as well see on picture below.

Okay guys, as a result of removing stubborn dirt and cleaning filter thereafter, this is how it looks now, not to mention my vacuum cleaner vacuuming like a diesel engine!

Thanks for your help geguymw and of course all others who contributed to this thread!

Sorry, forgot to attach picture!

I am glad that your vacuum cleaner is working fine again.

Your vacuum has the motor protection relief valve, which I talked about at the start of this thread, in the lid of the dirt bin. When airflow is cut off, this valve activates, to keep air flowing around the motor. That is where the "hissing" sound is coming from.

Happy Vacuuming!