Originally Posted by
donzidon
It could be the starter if the only thing you hear is the clunk - no crank movement and straining against compression. I had similar symptoms with a bad starter in my car many years ago.
However, really expensive things can happen if there is water leaking into the cylinders: bent valves, cracked block, pistons rusting in the cylinders while it is parked, etc. I think you really want to follow the advice above and check for water without delay.
Running well is no guarantee. I lost the motor on my 18 last year due to a bad cylinder. The yard found water in the oil after I pulled it out at season end. Motor never ran better than on that last run.