rattle/knock

[bucketmouth64]

I have a '98 explorer v8 and when I accelerate I hear a rattle/knock sound. Sometimes it is low sounding or no sound and other times it sounds loud. Can you give me some ideas what it could be and if the fix is expensive before I bring it in to the dealership to diagnose.

[Jeremy airjer W]

Can you leave it in park, raise the rpm's up and duplicate the noise or do you have to be moving?

Id there a particular speed or RPM that it does it at.

Do you hear it more as soon as the vehicle is started and then it goes away a couple seconds later?

[bucketmouth64]

I haven't done the parking thing. Sound goes away once I get to cruising speed and it re-occurs at times when I step on the accelerater. So the sound is not constant when cruising at a steady speed only upon depressing accelerater like from a stop or accelerating off a ramp into traffic or passing. I'll have to try the parking thing and let you know if I hear anything.

[Macgyver55]

Cracked flex plate? I've run into a few of those.

[Scott K]

They are known for cracked exhaust manifolds, could it be an exhaust leak?

[gunmunky]

I had an old Chevy truck that had the same symptom. It ended up being one of the plug wires had a pin hole in it and every so often it would arc out of the pin hole. Good luck.

[bucketmouth64]

Can you leave it in park, raise the rpm's up and duplicate the noise or do you have to be moving?

Id there a particular speed or RPM that it does it at.

Do you hear it more as soon as the vehicle is started and then it goes away a couple seconds later?

Tried revving the engine while in park and no noise.

[Jeremy airjer W]

Would you describe it as a fast hollow rattle with a low pitched?

[bucketmouth64]

I don't think it is a low pitch. Then again what you and I consider low pitch may be two different sounds. Sometimes it sounds low. Today, I accelerated and no sound. The fast hollow sound is somewhat accurate too. Can gas quality/brand have anything to do with it?

[Jeremy airjer W]

I was just going to mention that if you use regular unleaded on a regular basis try filling up with the next higher octane. If it stops than I think we can say that you are experiencing "ping". Which is a predetination of the air fuel mixture in the combustion chamber.

[bucketmouth64]

So is the fix to use premium fuel once in a while? Or is there something that needs to be fixed? What about an additive like sea foam?

[Scott K]

My truck will ping everytime I fill it with Holiday gas.