Vibrating when AC or heat on ???

98 ford escort,vibrates & shudder when heat or AC is on when in park or waiting at a light.While driving no problem,when i turn them off the problem stays for about a minute then stops,there are no trouble lights
or codes from the dash.
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I'm thinking it might be your rad fan.

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Bill opined in

Sounds to me like a low speed miss. Do a thorough tuneup.
Use your ears... isnt it that the engine is running a little slower with the
heater in MIX, DEF or A/C.. and THEN it starts missing?
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