I have had this sound coming from my engine bay(99 XJ 4.0), sounds exactly like a woodpecker pecking on my firewall. It only makes the sound while im idling. Ive grown somewhat used to it, however lately its been kinda buggin me!

Anyone else notice this?

One of my grandsons has the same engine and the same woodpecker.. His only knocks after a little spell idleing. I cannot hear it any other time.
I'm thinking it is a wristpin.. but I'm not positive of that.
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

The 99 xj, and most other mid to late 90's model Jeeps, Dodges and Chryslers have a pulse width actuated purge valve for the Evaporative emissions system. The valve will tick regularly, and won't change time with engine rpm. It sounds like a loud clock tick. Usually, you only hear it at idle, more so when the engine is first started cold. What you need to do is listen for the noise and open the hood, the valve is located in line with the evaporative purge hoses. Look at the emissions label vacuum diagram to get an idea where it is. Once you find it, put your hand on it, you will feel it working in time with the noise. What the factory fix was for the noise was to add a second rubber isolator to the mount. Some minivans have a insulating cover snapped over the purge valve to lessen the noise transmission. These parts should be easy to find at the local wrecking yard, once you now what to look for.

Make sure the valve isn't resting against any metal like the firewall or wheel well sheet metal, Even if it is resting on a hard line or wiring harness, as it will make the noise louder in the vehicle interior. You can add a bit of space to the valve with a couple loose zip ties holding the valve loosely to the original rubber isolator, which usually lessens the noise transmission.

Hope this helps! <img src="/forums/images/graemlins/patriot.gif" alt="" />

Thanks Marty, very interesting post. I intend to check my grandson's engine bay the next time I see the vehicle.
Big Jim <img src="/forums/images/graemlins/pfft.gif" alt="" />