Every once in a while when I give it gas, it seems to hesitate or stumble. That is accompanied by a thunking noise coming from near my feet behind the firewall. It'll continue to thunk until I let off the gas. What do you think it is? I thought maybe my clutch was slipping, but it was replaced about 1 1/2 to 2 years ago, and this has only started happening within the past 6 months. What do you think it is?