[ChevLoRay]

Listened to Click and Clack on Saturday....a woman called in with a question about a noise that happened when she switched on some heat/ac activity. She said the noise shook the front of her car so she would slow down whenever she was going to hit the switch. As the call progressed, C&C were asking questions, and assumed ( as I am sure many, many listerners did) that the front end of the vehicle actually shook at that time. Further comments determined that the shaking was making her "feel" as if the front end was shaking.

I said all fo that to say this: perceived noises are often louder and more remarkable than they are in actuality. We hear them and our mind goes into overdrive and imagines all sorts of possibilities.

In her case (an '88 Cherokee), they surmised that it was a blending door in the heat/AC system that was the source of the noise. I'd check the vacuum connections on the AC system and the mounting of the fan motor/blower assy. It could be that something has gotten into the blower area and the squirrel cage is contacting it as it turns. That could give out that sound you hear. As you drive along, whatever it is can bounce around and either contact the fan, or not. Try driving with the fan switch off, along with assuring that the defrost isn't on or the AC, either. If you know that they are off, do you still have the noise? If nothing can be assured, pull the fuse that powers the blower motor and see if you still have the noise. I'm gonna guess that something has fallen into the defroster vent and has worked its way down to the blower fan and that is the source of your noise.