Rattling noise
 

1991 K1500. 350. Automatic.

There's a rattling noise coming from the engine compartment, only when pressing the accelerator while driving. Does not happen if the engine is runnign while parked.

Can't look under the hood while driving. Makes it difficult to troubleshoot. Don't remember hearing the noise before, seems to be a new thing. Wondering if anyone had any thoughts on this?