weird noise?

I have a '99 1500 with 69,000 mi on it and its been making a weird sort of ticking noise. ex. say im ging 30 and accelerating to 40 it starts making it around 1500 rpm and goes away (or get quiet enough to where you can't hear it) when I let off the gas. What are some possible things that could be making this noise?

Ive Got A 99 Gmc 2500 That Makes A Weird Sort Of Rattley Noise Just Before It Shifts Sometimes. No Mechanics Have Been Able To Figure It Out, Although I Dont Think They Tried Very Hard. Since Its Not Consistent Its Hard To Show Them What Im Talking About. Anyway, If You Find Out What Yours Is Could You Let Me Know, Since It Might Help Me Along? Thanks For The Help In Advance.

Badphnx

If you run cheap gas in your truck (reg unleaded) try running a higher grade for a tank, like super, I know its not cheap, but it may kill the noise for that tank. If it does, odds are your hearing a lifter tick. if so, try running some fue system cleaner through the next few tanks, should help it out.

Take the truck out to a VERY dark place at night and open the hood. Have a helper gradually increase the throttle while you watch the engine. Keep your hands in your pockets. See if there's a wire arcing.