I've a "75 Dart /6 who's heater works fine (lots of hot air) but makes a heck of a vibrating racket at high speed only then the engine is running. When I turn the engine off and leave heater at high it runs as smooth as can be. I can duplicate the the vibration when the engine if off only if I close the vent. I can't figure out why having the engine running would make a difference with the heater fan motor. Any thoughts...please.

I will take a stab at it......

Closing the vent reduces the airflow through the squirrel cage fan and reduces the load on the motor. Less load and the motor spins faster.

Now with the engine running your alternator is charging and your heater motor is getting higher voltage. Higher voltage to the motor makes it spin faster.

Does this help?

Is it a vibration or dry bearings chattering? Makes a heck of a racket!