2003 Sub Forester with loud engine noise-knocking. Dealer cannot find problem. Any suggestions?

I have taken the car in for all its' scheduled maintanance like clockwork, and I use the recommended fuel. The dealer has actually gone over the car during my last two scheduled visits and cannot find anything that is causing the problem. The sound seems to be getting louder, and the car sometimes sounds as if it is going to stall. They have replaced the air filter and belts etc. They have put the car on the computer. The car has been taken out and driven by 2 of the mechanics. They have assured me the car is fine, but I just have this thought of having engine problems in the back of my mind. I have had really excellent luck with Subaru vehicles in the past, my last one had over 180,000 miles on it before I had to have any major repairs done. Can anyone give me any ideas what to do next. Would taking it in to a different Subaru dealer make any difference? I do know that engine noise tends to be louder in this model of vehicle. The noise is not affecting the performance. Thanks