ok so this is the first time i have every heard this noise but it made me really scared that something was wrong with my motor. so i was on my way home and i stopped half way to make get some gas and when i finished filling up my tank i started my car and i heard this noise and it sounded like a thumping noise coming off the engine so i popped the hood and watched the motor run but i couldn't tell what the problem was, so i had to just let it go and by the time i got to the stop light whhich was at most like 100 yards the noise started to fade away and then it was just gone. but it is still got me afraid that my motor may be close to blowing up. Now let me say this motor is in a 98 mustang and is a 3.8 liter v6 that has 200k miles and it is the original motor. So i;m a bit worried that the motor may be ready to expire. anybody has any past experience or something, that would be really helpful.