I came across a loud lennox pulas unit abot 1987....looked like a 90+ when heat comes on it makes a loud noise ...i throught the combustion motor might be bad ...so i took metal cover off ,removed insulatoin ......and it got real loud.....unit is putting heat out good..tooo the complaint was the loud noise and the unit shacks causing the fan downstairs to shack really bad....any one got any clue what causing the trouble......when i took metal cover off it looked like a muffler..... blower motor is not lose or going out....thxxxxxxxxxxxxxxxxxxx