Toyota has changed the design of two idler pulleys. When replacing them, buy washers to be installed behind the pulleys. Fast forward to time stamp 11:30 to see the change in the first video link. Washers are required to shield the bearings from contamination and align pulleys with the belt. The Car Care Nut says newer Toyota models were built with updated parts. I don't know when the change occurred for either 4th or 5th generation 4Runners.

The first video demonstrates how to diagnose engine noise. In this episode 2 idler pulleys are noisy. At 14:50, if the tensioner pulley is bad, replacing the tensioner assembly involves moving the air conditioning compressor. DIYers may replace just the pulley if the tensioner spring is OK. The tensioner pulley bolt is reverse thread. I didn't find a Toyota part number for the tensioner pulley. The second video shows how the replace the tensioner pulley bearings. An after market tensioner pulley is available.

Mechanic Tip on Engine Noise Diagnosis with Simple tools! - YouTube Car Car Nut Video

Lexus and Toyota tensioner pulley bearing replacement - YouTube 2010-2017 4Runner