Well in my truck for about the last year, I have been so [censored] annoyed by a knocking sound under the truck. Always thought it was a tranny cable vibrating and touching the top of the hump. It would only do it when at idle (in gear and ac on) so it would idle at the exact rpm to make the noise. Red-lights were terrible unless the radio was UP. Anyway, down to business, it turned out to be the heater core hose bouncing/hitting the intake plenum ever so slightly to make the sound travel through the heater hose and into the cabin. tie strapped the two heater hoses together and no more noise!