Every time I start my 76 911 (with the idle at 1000 rpms), I start to hear a stretching sound that seems to be coming from the fan/alternator area. As the stretching sounds, the rpms go from 1000 to 3000/4000. The fan belt is new/ and tight. I just bought the car recently and the previous owner told me this has never happened to him and that the car was recently tuned up at a Porsche dealer before my purchase. Here is a youtube video (not my car) but another car making the same sound.
Any help would be greatly appreciated -