It's hard to describe a noise in print, but let me try. In my '66 P I will get an occasional noise that I can best describe as a sharp, pop the top on a coke can sound. Not too loud and it seems to be on the right side and low when heard from the drivers seat. It can happen when cruising along in the 30-40 MPH range and sometimes when approaching a stop sign (thus downshifting or braking). There is no observable leak in any line or signs of blown fluids that such a pressure release sound would suggest. So, it sounds like a mini version of a trucks air brakes, very brief, not affecting any driving situation that I can determine. I've had the car for 13 years and this is the first I've heard it.