Q: My 2001 Volvo s40 males a loud thud every time I put it in drive. What could it mean?

The CV axle joints should be inspected for wear and/or damage. Putting the transmission into drive suddenly loads the joints and if there is looseness in those joints, as the play is suddenly taken up you can hear a thud. If the axles are good, it is possible that the thrust clearance in the transmission has grown beyond spec. That could be due to accumulated wear over the 160,000 miles you have traveled. Another common possibility is a worn, broken or mis-positioned motor or transmission mount. You should also check the fluid level in the transmission. Finally, if the engine idle speed is too high, that can cause harsh engagement and possible noises. I suggest you request a transmission noise upon engagement diagnostic and if the issue is not found there the Mechanic will evaluate the motor mounts, and cv axles, and also confirm whether your idle speed is correct.