Richard:

I am not into Mercs as much as some other cars, but had a 1986 190E 2.3 gas and just got rid of it @ 230K miles. This may not apply, so I apologize ahead of time.

I had a similar problem, and couldn't figure it out. I can't hear too well, but my wife was in the car with me and heard a hissing. I let it go for a while. It got a little louder to the point where I could hear it. The car idled a little rougher. Turned out I had a vacuum leak at the back of the brake booster (inside the car under the dash). I replaced the booster and the cruise control worked fine after that. I haven't figured the connecton between the two, but there might be one.

Metrikos