'88 560 SEC started making a strange noise while on the freeway - sounded like a slipping fan belt sort of noise. I pulled over as soon as I could, lifted the hood. By the way, the noise would only start when the rpms were 2000-2500. All the belts were taut and there was nothing obviously out of sorts. Got up to speed again, the sound came back, and running out of ideas, I thought I would try one more thing - I turned off the airconditioning - and that made the sound stop, the second I turned it off. So, what am I looking at - a bad air con pump? Any ideas?