Hi, my 2001 1.9T just started making a single popping sound when I start the engine cold. No noise when the engine is warm. The sound seems to come from the rear of the car (i have to try finding the spot with someone else starting the car). Is this a misfire? If so, what can be causing this? I just flushed my engine with swepco 503 flush (via a manifold vacuum hose as directed). The noise started happening a few days later. Is this related? Check engine was already on due to a CVVT reset code (camshaft reset valve error) which i was trying to fix via the flush. So I don't know if anything is being picked up by the cpu. Any ideas? Help!