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Mysterious Chirping

A couple of years ago I began to hear a high-pitched, metallic, chirping/chittering/rustling noise - sort of like tiny birds or insects - coming from somewhere behind me in my 1999 Toyota Tacoma pickup. The sound would only occur at speeds between about 30 and 55 mph, and would become louder after a long period of driving. Pressing down the accelerator would bring it on, but it would be loudest and chirpiest during the act of letting up on the accelerator. Coasting down a mountain road with my foot off the gas after a long day of driving made it very loud! I’ve had a couple of mechanics listen to it (one elderly car guy couldn’t hear sounds that high-pitched) and have sat my brother in the back of the truck to try to figure out where it’s coming from. The noise is still there, and can occur at lower speeds than formerly. In fact it’s there pretty continuously above about 15 mph, but still varies with acceleration as above. Any ideas?

Bad U-joint, loose heat shield or other such piece of sheet metal, worn/loose bed mounts are some possibilities.

I agree that it is likely to be a dry U-joint.
This is something that you should check out sooner, rather than later, because–if it is a dry U-joint–the inevitable result is dropping your drive shaft while on the highway, and that could ruin your entire day, in addition to doing some serious damage to the truck and the pavement.

Have you checked the brake pads? Have you checked the fluid level in the rear differential? Maybe a bearing could be bad? @cigroller u-joint is another possibility.