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