98 subaru forester engine knocking

there's a slight engine knock going on in my car. u can only hear it when you accelerate after stopping or if you're idling your car and u rev the engine. i gave it an oil change yesterday and the mechanic said that the engine was running with no oil. there is an apparent leak. problem is that there are no visible signs that shows any oil leaks. no oil leaks on the pavement or anything. i was shown where the leak might be, which is by where the oil filter is at, around that area. it was cleaned with brake cleaner, so the problem can be pinpointed at a later time. oil stabilizer was added during the oil change, but the knocking is still heard. what can be the worst and least case scenario that I'm facing here?