Sound doesnt matter, mine made a small hiss too. Yes there is a way to test compression, you LHS should have a compression guage which is just like a tire pressure monitor except that you thread it into the glow plug hole. We normally test the engines compression by turning the flywheel over. At the top of the stroke, for a used engine there should be a nice light pinch at the top. Not too light but still fairly easy to turn over by hand. This also depends on the engine as some have much tighter pinch than others. In my opinion, even if you are on a tight budget I would sell the old engine for parts (you can get around $35-40 depending on the run time) and buy a better one. If you dont want anything crazy I would suggest buying a nice .26 like a Mach or the XTM 24.7 which are both great mills. The XTM is really cheap $100 shipped off ebay. But not cheap in quality! One of the best .26's on the market today. The Mach I think is just a little bit more, good luck, Tom.[8D]