Sounds like a compressor.

I have a '91 300D 2.5 Turbo. About a year ago I noticed a repetitive sound like the discharge of air that an air compressor makes once its reached its maximum pressure. Originally occurring about every 15 seconds, over time, the sound has increased in frequency to about once every second.

From the underside of the car, I traced the sound to behind a small panel located roughly under the back of the front passengers seat. It's a valve but I have no idea of the value's purpose or whether it represents a serious situation. Everything else on the car is working fine.

Is this a DIY project?