I've had my Englander 25 PDVC for 5 days now and am noticing a vibration that sounds like a low rumble sound. It happens periodically and it seems to be happening at mid to high level settings. I'm using one tee after 18", then to an elbow at the exhaust. The OAK is installed according to the directions. I have the unit sitting about 2" above the floor on a solidly built platform that has Hardi Backer Board on top. I've been cleaning the unit each day. When I turn it off, the blower runs as it should, but I don't hear a rumble. When I hear it, it sort of sounds like the exhaust fan is rumbling like a bad bearing, but that's just a guess. Someone else on this site had a similar problem with a used one. Mine is new. Any suggestions?