[ctrlz]

Originally Posted by Efi69Cam

OP if the pump is relay controlled you can easily test if it’s stuck on by pulling that relay. Also you can jump the terminals in the fuse box to turn the pump on and off to verify the noise.

Underhood electrical box:
Pump fuse is F14
Pump relay is K4

[I don't want a mustang]

I got a tech to check it out. The sound was indeed the electric brake booster vacuum pump. Not knowing what it was and trying to detect the problem from the engine bay, I didn't realize I could recreate the problem by pumping the brakes a few times. It didn't seem related to brakes as it would happen with my foot solidly on the brake during idling. He edumacated me on how the pump works, so now I know better.

The tech said that the pump was operating normally. I just didn't expect to be able to hear it from the cabin. It seems sketchy like the pump is about to fail. I could always get one from a junk yard, connect it to power and compare it to my existing pump.

My dad always told me you'll go broke or crazy chasing sounds on a car. I just thought I knew better. Thanks everyone for your help and input.