Help -- mystery! I couldn't find anything exactly like our situation, so would really value master plumber input (especially as our local guys aren't returning calls). This is a long post, but we wanted to give you all the detail.

Problem 1: About 1.5 years ago, our water pressure/volume decreased significantly throughout house. We put in a whole-house filter which helps, but filter needs to be changed every couple of months b/c it totally fills up with dirt/sludge from the water main.

Problem 2: Right after anyone stops using the shower, there is a buzzing/humming noise, kind of like a generator. It used to happen only when the shower on ground floor was used, but now it happens with any shower. This started after the filter was installed.

House details: 4-story rowhouse in NYC, completely replumbed when we moved in 4 years ago. New water main (1.5"), new risers 1/4". 2 units: ground floor with 1 bathroom, W/D, DW; 3-story house above with 2.5 bathrooms, W/D on top floor, DW. 2 water heaters in basement, one for each unit. W/D on top floor is HE, and we have water-hammer arrestors on it.

It seems like the sludge problem developed after we moved in. As though dirt got into the system from the city unrelated to our renovation? We're assuming it's not normal to have to change the whole-house filter every couple of months. (To be fair, we are heavy users, between 6 long-showerers and a garden to water.) After we change the filter, we have good pressure/volume throughout the house, until it plugs up again.

The humming noise is disturbing. It actually vibrates in the floor, more so that in the wall with the risers. We wonder if it could be related to draining the system when we change the whole-house filter?

Apologies for the long post. We look forward to hearing suggestions and ideas!