Pop under Hood

I hope someone can help me with this....

I have a 1999 Gmc Sierra....

Sometimes when I start my truck it takes what seems like forever (10-15 secs) to fire up. It sounds like it wants to fire up but doesnt. Other times it fires right up like normal.

Today it was doing its normal take forever start up and I heard a large "pop" come from the engine bay on the passenger side.

The truck finally fired up and everything was cool...no change in operation.

The truck drives great..however I can feel a slight shimmy or shutter when the truck is ideling....but only slightly.

What could this be?

I have recently changed my air filter, gas filter and oil after I started noticing that my truck was taking quite a long time to start...but nothing has fixed the problem.

Please help guys/girls

Xtreme

Manifold leak? Someone with a diagnostic tool should be able to test for that.