91' 4 Runner burping sound when drops to idle

Just completed timing belt, waterpump, etc replacement, have 330K miles. Replaced number of bad vacuum lines and old hoses. Runs good, but makes burping noise when it drops back to idle. Could that be a vacuum leak, or maybe air still in the coolant system? Sounds is coming from the back side of the engine, on passenger side...at least that's how it sounds.

 

I would say you got air in the coolant system somewhere.

 

My 80 did this after I replaced the wp. Air bubble.

 

Bleeding several times hasn't solved the issue and during a test drive, the vehicle is making very little power. I'm confident the timing belt replacement was done correctly with proper timing. Could replacing vacuum hoses have created a problem...maybe a failed component was getting "bypassed" due to split vacuum lines? We also put a new O2 sensor on...could that be throwing off the mixture? We ran the diagnostic and just get constant blinking. Any ideas would be much appreciated. Thx!!

Also, adjusting the idle mix screw as very little effect.