My 126 does something that sounds like what you're describing.
The thunking/clunking sound sounds like something in the front left suspension is LOOSE. It only occurs when going over a *sharp* type bump, ie, railroad tracks or a bumpy/rocky road....something other than a smooth transition bump like a driveway curb or a smooth speedbump.
Everything has been checked up front except the sway bar bushings. This may be my problem as well.
1985 300SD - 'Grace' (198K mi.)
2010 Honda Fit Sport
2013 Honda Accord Sport
1996 Lexus LS400 (Retired)
1995 Ford Contour SE (Retired)
1976 Porsche 914 (Sold)
1972 Datsun 240Z (RIP)