90 T-bird Engine knock

I just recently got a free tbird from my friend. The car ran fine, I first noticed a knock that was so quiet you could only hear it from underneath the car, not under the hood. No tach but I'm guessing that it is somewhere around 2500 rpms only. I drove it around for 3 days with no problems, but when I went to canby, on the way back I could hear it quite louder. I stopped by taco bell and put it in neutral and pushed on the gas till the point where the knock could be heard, and it has gotten quite loud, loud enough now to be able to hear from under the hood. And from inside the car with the windows up.

It's a 1990 for thunderbird with a V6 3.8L (not SC or LX) with around 140K miles on it. The engine oil level is good, not new but level is good. When I let off while it is knocking the knock speed gets faster for 3 or 4 knocks but as soon as the rpm's lower it goes away, it doesn't knock at high revs or idle, only around that area of RPM. I don't know ALOT, but I was told that it has hydraulic lifters and that it wasnt the problem, I hope it isn't a main. Any help would be appreciated.

anrchy