Based on a comment posted by Christian (96 Runner) about Jason Briggs at Tempe Toyota, I spoke to Jason today, got put through to a service tech, who solved the problem for me. It seems there is a plate on the firewall side of the air suction valve (ASV), and this plate is held on by 4 #10-32 screws. One had fallen off of my ASV -- I replaced it and no more popping, rattling or gurgling! In my experience, it is rare that a dealer service advisor and/or tech will be as cooperative as these two guys were. Now my 4Runner does not sound like an out-of-tune bagpipe mixed with a rabid baritone duck.