Weird "chirping" noise coming from the rear when driving....?

zup peeps....

ok, I know that the chirping is not a bird stuck in my muffler or the trunk but there's a chirping noise coming from the rear whenever I am driving. Past 40mph I get a constant chirp, like 50 birds stuck in the trunk and their all hungry!!! Whenever I am on the highway I have to crank the radio high or I'll loose my mind with the chirps, even with the radio I can still hear a faint noise of it. But as soon as I start driving it doesnt chirp, it's like eventually starts, maybe after a slight bump on the road, or just because.

I've also noticed when I hit a small bump on the road, like leaving a parking lot going 15-20mph, bump = one quick chirp. It's weird. I've debated about taking it to a ford dealership to have it looked at but I dont want to spend $$$ for them to say is a loose screw or something. Talking about screws, I even screwed on the spoiler even tighter (thought it could be loose and making noise with the wind), thought that would fix it...NOT!

So even though I know it sounds hallarious (which in a way it is!) it drives me crazy and it's very annoying. Maybe the rear shocks? maybe the gas tank is loose? What can it be? any help would be appreciated!!!

Thanks!

ps- after a quick google, this page for a fiero might better describe what I hear, but instead of happening for a second it just continues to chirp. 2nd paragraph HERE