This past winter I starting noticing a LOUD thudding noise that vibrates my whole house. At first it would only last 15 - 20 seconds. As it was seemingly coming from my basement where all the mechancial "guts" of my home are, my first guess was that it was my hot water heater vibrating from scale build up. I flushed my hot water heater and it didn't resolve. My next thought was that it was the basement bathroom vent. I tested that, and elimnated that option. The sound/vibration has continued and gotten stronger and lasts longer and longer now. However, it doesn't happen daily or at a regular time. It happens regardless of whether my HVAC system is on or not. I also haven't been able to determine any correlation between specific faucets or plumbing fixtures. In googling to figure out the problem I came upon "water hammer", and if this video:is an accurate representation of what "water hammer" sounds like, then I strongly suspect that's what's happening. However, as I cannot induce the sound "on command", I'm hesitant to call a plumber as there is no way to know if the sound will occur for him/her to diagnose. Given everything I've read about water hammer and its indication of potential major problems, and the fact the noise is lasting several minutes now and the vibrating is getting strong I'm pretty concerned. Any guidance on how to try and reliably induce the sound and/or confirm it is water hammer would be very greatly appreciated!!