Interior Noise - Like A Water Pipe Bang/Vibration
 

1984 5.0L with a pipe bang/vibration/pumping sound in the interior on the passenger footwell/door area. Sounds like fluid moving through a pipe or hose that is vibrating.

Sounds is not related to engine speed and is most notable at idle. The sound changes in frequency, ramps up and down and goes away and then can come back. The sound comes and goes, hot/cold makes no difference. You cannot hear the sound outside of the car at idle. Recent exhaust conversion to duals made no differences so most likely unrelated to that change. Have grabbed hoses and other fluid lines and cannot seem to isolate the cause of the noise.

Any ideas?

heater core could be leaking into the HVAC box, or has become loose somehow and is whackin around in there.

plus 1 on the heater core

Thanks. I will look there. I hear gettng to the Heater core can be a real pain!

The 4 eyed cars are easier to get to than the 87+.