Any thoughts on what this is - brakes wearing out? Thanks!
Its more likely that you have a bad tire. Better get them checked, one of them is probably coming apart and could fail catastrophically at any time!
This sounds like a tire problem and it will not cure itself. The only change that is likely to take place is a blow-out and when this happens at high speed the outcome is not usually good for the driver and passengers.
Have all 4 tires checked and be prepared for replacement of all 4–assuming that it is an AWD version of the RAV4. If you read the details on tire replacement in your RAV4 Owner’s Manual, it undoubtedly tells you that you must have 4 tires that are matched as to brand, size, and amount of tread wear.