I have a 1996 Ford F-250 PSD.

My truck idles at about 600 RPM. Every once in a while, at random times, it drops to about 500 RPM and makes a grinding noise. The whole truck shakes and it feels like everything is about to fall off. It only makes the grinding noise for about 5 - 10 seconds tops then goes back to its normal RPM's. It never stalls but it sure sounds like its going to. Like I said, it does it whenever it wants. It doesnt matter if I put it in neutral coasting down a street or its just idleing in the driveway. Thanks in advance for all feedback

 

I would look at the A/C compressor. Turn the A/C on and get out and watch it engauge and disengauge. That's the only thing I can think of that would be intermittant like that.

RubberDuck

 

Where is the A/C compressor located?

 

If you are standing at the grill, it is the upper right. The highest thing on the right with a pulley on it. With your A/C on, but not on Max A/C it should cycle on and off. Just stand out there and watch it. If it cycles on and off and you don't hear anything, I don't know where to go next.

RubberDuck

 

Sounds like the noise could be the dual mass flywheel going bad if you still have it. They will shake and make an awful noise. Check the rotational slack in it, it should be no more than 5/8".

 

How do I go about checking that? And thanks for all the replys so far!

 

Take the dust cover off the bell housing (on the front of trans), put the trans in neutral and rotate the back half of the masses opposite of the front one. You should not be able to move them further than 5/8". Although my friend replaced his and he didn't have excessive slack but the springs were about to sling everywhere. I guess it would have gotten a lot noisier the longer it went. Guess he caught it in time.

Maybe this will help.

Good Luck!

 

I vote for the AC pump going out too. It's the only thing that will cycle on and off plus make enough drag to slow down the engine.
Note: Even in the wintertime and you are using defrost the AC will turn on. It's done that way by design because it's great for extracting moisture out of the air. It clears a foggy windshield much faster that way.

 

With it cycling that does seem suspect.

 

Thanks for all the help so far. Ill look into all your suggestions.