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Thunk when Auto Trans Shifts

I drive a 2006 Subaru Forester with about 77,000 miles on it (Automatic Transmission). I’ve always been able to feel the car shifting (as I can in most automatic vehicles). Recently I’ve started to hear a “thunk” accompanying the shifting, and a pronounced slowing when the car shifts. I’ve only noticed this when the car is cold. The sound is similar to what you would hear if you ran over a small animal carcass, but it is coming from the center-front of the car, not the tires. I only notice it happening when the car is cold (after sitting outside on a 35 degree day while I’m at work, for example). Is this new behavior due to age? Should I just let the car warm up more? Or is this a symptom of a problem that will only get worse and more pronounced? Should I take it to the shop to have it looked at, or is it an easy fix that I can do myself?

Also, admittedly I don’t take the best care of the car… I change the oil regularly, but I don’t do the scheduled maintenance in the Owner’s manual. I only change the other fluids (transmission, differential, etc.) if it looks dirty to me. Is my laziness just exacerbating the situation?

Thanks in advance for your help!

One more thing: I only notice the “thunk” and slowing when my car shifts into/out of 3rd or 4th gear.

Yes, your laziness (cheapness) is exacerbating the situation. Subarus, as you may be about to learn, don’t take kindly to neglect. There’s a reason the owner’s manual recommends changing the fluids you mentioned at certain intervals, and “looks dirty to me” is not the criteria.

At 77K miles, the transmission fluid should have been replaced twice, and the differential fluids are probably due, if not overdue.

I suggest you have all recommended maintenance brought up to date and cross your fingers. Maybe you’ll be lucky and there won’t be any significant damage.

The Subaru AWD system, at least in automatic cars, relies on the automatic transmission fluid for proper operation of the transfer valve and clutch pack. Lack of proper maintenance results in increased wear on these components, which are expensive to replace.

“Pronounced slowing when the car shifts?” That doesn’t sound good.

What about things like spark plugs, filters, etc? Do you replace them or ignore them?

If you have replaced the differential and automatic tranny fluids at 77,000 miles and changed the oil according to schedule, you can hardly be classified as a lazy or negiligent owner. If you were a few thousand miles late on a fluid change you caused no damage.
It’s likely you have failing cv-joint. Take the car in and have it tested. the repair is around $300.