The car runs great and has over 210k miles. The only thing that I can't figure out is that under hard acceleration, around 20-30 mph, the car chatters. If I let go the gas, then the chatter stops. So I think it's related to the engine but not sure what is the culprit.

Anyone has any idea? Thanks.

Sounds like a bad front axle to me. Figuring out which one is the fun part. We have had great success with "new" axles from Napa. I don't recommend "reman" when it comes to axles.