Q. I own a 2016 Subaru Outback 3.6R with 76,000 miles. For the past few months, I’ve occasionally experienced an unpredictable thump while stopped at a red light or in traffic. The sensation feels as though another vehicle lightly bumped me from behind. A Subaru dealer was unable to duplicate the problem and said it was unrelated to the rear wheel bearings they replaced. Because my vehicle was covered by Subaru’s extended CVT warranty, I opened a case with Subaru in case the issue later proves to be transmission-related. More recently, a check engine light led my local mechanic to diagnose a leaking fuel injector and a misfire. He replaced all six spark plugs and changed the oil, but the thumping has continued. I’ve read that the air conditioning system could also cause a similar symptom, but since this is my son’s car and the A/C runs almost constantly during the summer, it has been difficult to test that theory. What are the most likely causes of this intermittent thumping sensation while stopped, and where should I focus my troubleshooting efforts? Would a transmission fluid change help?

A. Based on the symptoms, the CVT transmission remains a possibility, although there are other potential causes. It is possible the variator, the internal variable-drive mechanism, is occasionally hanging up. I doubt a fluid change will solve the problem, but it may be worth trying. Although CVT transmissions have improved over the years, regular fluid changes are extremely important. Changing the fluid every 30,000 to 60,000 miles can help prevent belt slippage, overheating, and premature mechanical failure. Other possible causes include engine or transmission mounts, air-conditioning compressor........

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