Thanks for the replies. It's 420 SEL that's making the noise.
The mechanic rechecked all the nuts & bolts but still the noise is there. BTW, the mechanic assured me that it's not a safety issue. However, when I go over several consecutive cracks/bumps on the road, the noise bothers me a bit.
Could this thumping noise be related to a rope tightening noise when I turn the wheel while in park?
I appreciate any suggestions.