At high rpms, whether in gear or idling, but most of the time while in gear and right before a shift, I hear a rattling noise from the center of my dash. It's slightly to the passenger side. Slightly toward the lower dash area (but not the floor hump).

It's not a heavy sound, but more like sheet metal rattling, but for the life of me I can't isolate and find it.

If I had an old cable style speedometer I'd say it's the cable too dry. That's what the sound is like. Or even like a small rattle from a loose fitting under the dash. But it does get a little loud at times, so it seems engine related even though it's coming from the dash area.

Any ideas here? I don't want to ask my shop to start a wild goose chase just yet.

 

Bad engine or trans mount maybe? They can cause somewhat of a rattle noise if they get too bad.

 

Wow !! This could be almost anything... It could be something as serous as what NullHead mentioned to something as simply as a dime fulling behind your dash pad or down your defrost vent and landing in that sweet spot to cause a rattle

 

Does your splitter work on your HVAC?

What I mean is, when you have your air set to blow out at your feet or at the floor, does it actually blow where you ask it to?

These model trucks are notorious for the plastic blade to pop out of it's mounts and it will just sit inside of the vents under that part of the dash. I'm thinking that maybe the high rpms are rattling it around?

 

That is exactly what it sounds like, and I'd wondered if anything might be in there. I'm going to check asap. Is there any way that I caused the problem by putting a new fan on two years ago? The fan is at the far right.

 

This may be of some help? Different year but similar assembly

http://www.justanswer.com/chevy/54sl...silverado.html