ABS howl?
 

2004 Pacifica AWD exhibits an intermittent howling noise under hood after stopping at a stop light after 3-4 seconds with foot on brake and auto in "D", noise then quits after releasing brake. New pads on front & rear. Brake fluid is adequate but appears to be a little dirty. Brake pedal appears to be a little soft. No ABS light comes on. Can this be checked with a code reader? I recently had the EGR valve replaced when the plug popped out. Any ideas or thoughts?

Noise quits after releasing brake? Sounds like a leak in your brake booster.

My wife informs me that she felt a vibration on the forward floor board thru her bare feet when the noise occurred and then it quit when the howl quit. If there was a leak, why would it not do it all the time? Thanks for your input, I will check that out.

OK, there was a howl. Now there is a howl and a vibration under the feet. Shew. Now you lost me.

The howl now sounds like a "Foghorn". Dealer mechanic was able to generate the condition and by touching the AC Filter/Drier felt the vibration with the foghorn sound. They have never had this condition before. I left the vehicle with them as the warranty will cover whatever else they find. So far the compressor & clutch are ok. Brakes are ok. What would cause this unit to do that??

Well, that pedal issue sounds like the ABS pump running. May be some air in the system (as in a leak/pressure bleed off or internal seal) or there is a pump and/or module electrical malfunction.

ABS howl? Is now A/C Foghorn sound
 

Dealer was able to determine that this loud foghorn sound occurs when the high side PSI reached 260psi until the A/C fan turns on and the noise was coming from the Receiver/Drier. They recovered the system and replaced the Receiver/Drier and Thermal Expansion Valve and re-charged the system. Hopefully this will end this saga.