This motor WAS sounding perfectly quiet and normal, until I gave it an oil change. When I was done, I fired it up, and it made a loud knocking sound that sounds very serious, like a bad rod or something. I can't tell where it is coming from, and it is so scary, I hate to run the motor for fear of making it worse. It is a constant bang, bang, bang, and I have not revved it beyond idle. It starts easily with one kick, as usual, and it turns over smoothly with the kickstarter. Before I start tearing the motor apart, I sure could use some advice from others with similar experiences. My mind is burned up with thoughts of a major catastrophe. Help!