'97 E420 - couple of months ago AC compressor starting making a clunking noise for a second or so after clutch engagement. Now, with the HVAC fan at low speeds, the clunking noise can also be heard during moderate braking; with the vehicle at a standstill the noise stops. Otherwise, the system cools well.

Is compressor failure imminent? Should it be replaced now, to avoid the potential for contaminating the refrigerant lines with metallic debris?

Have a competent automotive A/C tech listen to it. Your may just be now noticing something that has always been there. It is not unusual for the comp to be louder when it first starts turning as it is working with a much higher suction pressure. It it is quiet when running at idle and above--it is probably normal and OK. If it is failing--its too late to avoid having to flush out the line from the compressor to the condensor, the condensor, and line to the filter drier (which has to be replaced in any case.)

Purchased this car 4 months ago with 84K miles, compressor noise was evident at that time but only for about a second after the clutch engaged. Now, at 89K miles, with the compressor running, the noise is occurring during braking, as though there is excess free play in compressor's internal components. A review of the VMI shows warranty AC work at 45K miles but no details are available. Have another '97 E420 with over 115K miles and its compressor is quite at all times.