Top end knocking when hot

I have a 99 Intrigue with a 3.5. Friday, it started knocking while we were running down the interstate. I found the oil was low (wife's car :banghead: ) and a little more than a quart put it back to normal. I had a mechanic friend listen to it and he wasn't sure what the problem could be. He did explain that if it was in the lower end, we probably wouldn't have made it as far as we did, but he didn't rule out a problem in the lower end. The knocking sound was pretty loud. If I give it a little gas, the knocking disappears, but returns when I let off the gas. I drove the car quite a bit after topping off the oil that day, figuring if it was junk I'd rather find out while close to home, but nothing changed.

After sitting unused for a couple of days, I started it up this morning to come to work. It was absolutely quiet ... not even a tick. About four miles down the highway I pulled over and listened again. It was ticking. About ten miles later, it was knocking again, just like it was on friday. The oil was changed about 1500 miles ago (four weeks) and looks perfectly clean. The motor has 150000 miles on it, but has always run perfectly and up until now has shown no signs of wearing out. I'm not even sure why the oil was low, unless the oil change place left it that way. It doesn't seem to burn oil, has no leaks, but since it is an Intrigue I guess it could have just disappeared as seems to sometimes happen with Intrigues. :screwy:

Any suggestions would be appreciated. I'm hoping it's not ready for the bone yard. I really like this car.

change to synthetic oil (Mobil 1, Amsoil) and hope for the best.