I would get an independent evaluation of the system and based upon that report decide what to do. There are system cleaners that would allow the existing piping to be used for the 410a refrigerant. It is definitely best to replace, but in this situation cleaning may be best.

I guess you could also sue for loss of use of your home, to force the owners who are aware of the problem and worked with you to be good neighbors to address the issue.

Good luck!

Have you discussed the problem with the condo association? The condo bylaws may have rules about excessive noise after 10 pm.

ok. thanks Jackfre and Mike_home. I contacted the home owners association 4-5 times over the last year, and finally they gave up and told me to contact the rental property management company because they said HOA doesn't have enforcement power on this. It's very frustrating.
I am considering putting spring vibration isolators underneath as the first step and see what happens. Then will get an evaluation to see how much it is to replace the entire R22 to R410 system (maybe flushing the piping) and see how much that is and offer to cover 50% of the cost. If the owner refuses, I am considering law suit....but not sure what are my chances.

In your desire to help yourself, by adding vibration isolation, you are probably assuming the responsibility for the units operation. Do not touch that unit or it will become yours. All yours! Call your attorney and discuss it with him. The owners are aware of the problem as they were willing to work with you by modifying their use of the "problem". I know how I would handle it, but, well, that is just me!

Hi Jackfre, that's very good advice. I am going to buy those vibrator isolators and have the repairman put them underneath for me - the repairman is dispatched by the property management company of the rental unit. I assume that would be fine.....

Just guessing on my part, the vibration isolators will help to reduce but will not completely eliminate the noise.

The thing about noise or vibration on equipment is you don't know exactly where it is coming from. Is it the compressor? Is it the line set? You appear to be determined to get into this unit without getting legal advise. I think that is a mistake as even having the associations person do it, it is still coming at your direction. You assume responsibility. I think that is the way this goes. Good luck!

I am sure the vibration noise played a big part. When I stand bare foot on the carpet underneath the air conditioner, I feel the vibration. Also the repairman said that the compressor blanket wouldn't help because the noise is the vibration. However, the repairman doesn't really seem to care about trouble-shooting the vibration. I had an appointment with the repairman next Wednesday and will take a look and discuss the options. By the way, it's not the association person I am dealing with. The repairman deals with the rental property management company, which in turn acts on behalf of the downstairs unit owner when the repair cost is under certain amount. Given that nobody in this mess has the incentive to solve the issue but me, I am driving the process and hoping to come up with a solution to alleviate my problem. Once I have an agreed upon solution with the repairman, I am going back to the rental property management company to tell them what we are going to do, and hope that the owner will agree. After all this is a low cost solution. It would be outrageous if the owner doesn't agree. I have been sleeping on the floor on my 2nd bedroom on a futon for a week now. Yesterday I attempted to stay in my own bedroom with earplugs again but I could still here the ringing/grinding sound which gave me a headache. Eventually I was driven back to the futon on the floor.

You can use the R22 copper lineset with R410a refrigerant if it is throughly flushed out. Request a copy of the HOA bylaws and review them. They should prevail over any rental company agreement.

thank you sktn77a. That's good to know that I can still use the lineset after flushing out the R22.
Re: HOA bylaws: It has no teeth and no enforcement power - that's why I have come to this point.....
Anyhow, the Property Management company is now in contact with the owner on my proposal to put on the spring vibration isolator and sound blanket, it looks that they would say yes after confirming this will not harm the operation of the air-conditioning unit. I will find out next week.

What happened and what was the resolution? I’m having the exact same issues

I put in the vibration isolators , it helped to some degree, but not all. eventually their air conditioner died and replaced with a new one with a hard plastic platform underneath. I also asked the HVAC people to put in the same isolators. But the biggest help, it turned out, was that the HVAC people added some sort of isolation on the air exchanger that was mounted beneath my floor. So with isolation both on the top and bottom, the noise diminished so much that I can fall asleep now. Another unit air-conditioner on top of me was replaced also, a hard plastic platform is also used.