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Subaru Tire Rumbling

When I drive my 2001 Outback, I feel/hear a rumbling in the rear tires when I make a turn at 20 mph or less. It’s like I am going over rumble strips. Any ideas?

I think that you are describing the symptoms of “torque bind”.
This results from damage and/or excess wear and tear of the center viscous coupler/clutch packs.

This type of damage/wear is usually caused by failure to rotate the tires on a regular schedule (every 5k or 7.5k miles), or by having tires that are mis-matched as to size or tread wear, or from driving too far on the temporary spare tire.

Are you the original owner?
What can you tell us about the condition/wear/size/brand of all 4 tires?

Hmm. Looking back, this start in the last couple of months and I realize I had the tires rotated at the end of June '10…Noise started after that. I bought 4 new tires in March 09. They were P225/60R16 VIVA 2 and I had 135K on the car then, now have 170K. In looking at my oil changes, they list the tread depths, and in April 2010, they were 7/32, 8/32, 7/32 and 4/32 for Driver Fr, Driver Rear, Passenger rear and Pass Front.

That 4/32" seems really odd. It may be causing your issues if currently installed on the rear axle.

However before you jump to driveline issues make sure the tires are not the culprit. Rotate them back and see what happens maybe. Also reverify those depths. The 4/32" is way out of spec vs the other tires.

With differences in tread depth that extreme on a set of tires that are the same age, I have to suspect that the tire rotation in June, 2010 was the only rotation, or at least that the tires were not rotated often enough.

Subaru specifies every 7,500 miles for rotation, so the tires should have been rotated at least 4 times since they were mounted, preferably 5 times. Was that done?

Also–it is very possible that your wheel alignment is “off”.
Was the alignment checked at any point after purchase of those tires?

Anyway, the horse if out of the barn, so to speak.
The extreme differences in tread depth on those tires has likely led to damage to the center viscous coupler/clutch pack. Take the car to a mechanic who is very experienced with Subarus in order to verify this situation. If he confirms my suspicions, expect to pay something on the order of $600-700 for repairs.

Also, you will need to replace the tires with a matched set, have the wheels aligned, and rotate the tires every 7.5k miles. If you do not do all of that, you will wind up blowing the new center viscous coupler/clutch pack after a few thousand miles.

You might be able to get an identical new tire shaved to 8/32ds, saving you the cost of three more tires. Get an alignment and rotate more often. See if that helps the rumbling to go away.