Air conditioner smells and a mysterious squeak

cabin filter prior to spraying a disinfectant would allow a greater rate of passage and eliminate the possibility of the filter becoming temporarily saturated with the product. On some vehicles, removing and replacing the filter is an easy, do-it-yourself project, while with others, the car is seemingly built around the part.

Your BMW squeak is a bit perplexing. It sounds like this is a repetitive sound as opposed to a single one. BMW released a service bulletin in 2009 (SI B33 04 08) addressing the possibility of a single clicking or cracking sound during take-off or torque change. The fix is to lubricate the differential input flange-pinion shaft splines. Replacement of the pinion flange seal and collar nut retaining plate are also specified.

I’m thinking a repetitive noise that is not affected by braking might be caused by a worn or dry constant velocity joint in one of the rear axles. If inspection indicates a rubber boot crack or tear, it’s possible lubrication has escaped and dirt has entered the joint. You might also try, with the vehicle safely elevated and supported and fresh off a long drive, yanking and twisting on the joints, checking for looseness and checking for elevated temperature at each end of both axle shafts, which indicates excessive friction.

Low differential fluid isn’t a good thing but would have no effect on wheel bearings as they are permanently lubricated and sealed. A noisy wheel bearing should sound off differently when turning to one side or the other. Noises related to low or contaminated differential fluid include a 20 mph and up drone/ whirring noise (carrier bearings), a whine on deceleration (pinion shaft bearings), and a whine during acceleration frequently the result of a ring and pinion gear problem. By Brad Bergholdt I’m curious, when using Lysol spray to disinfect a foul-smelling system, should I take out the cabin air filter? If the Lysol hits the air filter before reaching the air conditioning system’s evaporator, will some of the chemical wind up clogging the filter? In another matter, I have a 2003 BMW Z4 2.5i with an automatic transmission and about 165,000 miles on it. Upon initial acceleration (when the engine’s torque revs up), the car’s rear end lets out a high-pitched squeak that then disappears once I’m cruising (and torque is reduced). You cannot hear it with the windows up and the noise does not correlate with braking. I replaced the differential oil, which was low and somewhat dirty, but that did not get rid of the noise. Could the rearwheel bearings be the cause? — Peter Schmidt

Peter, you bring up a great point about the cabin filter. The majority of current vehicles utilize a paper, carbon, or paper-carbon filter to keep dust, pollen, and other debris from passing through the heating, ventilation, and air conditioning system. I should have mentioned that renewing the filter could help reduce offensive odors as it may contain bacteria in addition to that found deeper within the system. Air conditioner-specific disinfectants can also be purchased at a neighborhood auto parts store and may work better than Lysol. Removing the