I have an '83 300SD that sounds fine with the heater off, or with the air conditioner on. When I turn the heater on it's fine, or the defroster on it's fine, but when the heater or defroster is on and I accelerate, it makes a horrible noise. I only bought it in August, never had this noise in the summer with the a/c on, and this is the first time I turned the heater on while driving. Drove 40 miles home just fine, up steep hills, but with no heat and the back windows cracked for a defroster. I searched tech help and couldn't find anything similar. Any ideas? We would love to fix it ourselves and save the $$$. Thanks!

Rikki

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1983 300SD

The additional load just might be making the alternator belt slip and squeel. If you can turn the alternator pulley by hand the belt is either too loose or glazed. Nice cheap fix if the problem.