I just had a new power vented water heater installed (HTProducts Phoenix Light Duty) in our crawlspace. The contractor that put it in used part of the existing 3" concentric vent from the furnace we removed used.
The issue is as the exhaust fan gets up to higher speed a loud hum starts in the vent that sounds a bit like a (albeit quiet) fog horn. This noise is nothing like the fan noise close to the water heater. It's low frequency and loud enough to annoy us in our bedroom on the second floor and most likely our neighbor.
I am fairly certain the issue is to do with some kind of natural harmonic happening in the exhaust vent and this is reinforced in that if you momentarily cup your hand over the vent exhaust (blocking the air flow) the hum stops and it goes back to the quiet expected noise for maybe 10-20 seconds, sometimes longer, depending on what the fan is doing then.
Has anyone come across this or have a solution? Total length of pipe isn't really an option as we can't move the water heater position though the length of where the smaller pvc pipe from the water heater exhaust joins into the 3" may be.
EDIT Thanks for the suggestions. Further info: The fan is built into the water heater unit and as it is new and is controlled by the water heater cpu and I don't want to void the warranty messing with the fan is out. The manufacturer said they haven't heard of this. Based on the noise and feeling the vent pipe I think the standing wave is caused by the fan beating the air and there is minimal vibration on the vent itself. Also the hvac guy that put it in has three ninety degree angles in the 2inch pipe before it goes into the 3inch which then goes straight outside.
UPDATE So after installing 3inch pvc pipe the whole way from the water heater to the concentric vent and still being puzzled at the hum the contractor started going back over everything and after adjusting the combustion mix it appears the hum was caused by too lean a mix.