I have a 1986 420 SEL. The car has been great lately. But something new. I here this thumping by the shifter and dash above it. I know it is related to vacuum because it happens when i push the gas and quits when I let off. No vacuum--no noise. Over a short period off time it goes from tolerable to intolerable. It gets LOUD. It also has a crescendo. It ends in a loud pop and a real nasty sound--like something is bad. After that trip home I took everything apart and was going for the vacuum lines to the heating unit. I dug and i did find one undone. All was quiet and forgotten. Then it started again. I took everything apart again so I could get access again and could not see anything. So I left it apart and drove that way. It is the crash sensor box. I can feel the vibration in it as i drive. The same thing happened. It got louder and louder to where just cant stand it anymore and then it does the "everything is going to hell " sound and is quiet again. This takes a couple of weeks to go through this cycle. I looked at the archives and what little there is doesnt offer me enough pieces of this puzzle. There is a vacuum line to the crash sensor but is large and from the archives I am led to believe it is conected to the vacuum pump in the trunk. The sound is linked to engine vacuum. The archives also stress danger danger danger when working on the SRS system. Thanks to anyone with something to add, I am confused.