Air Pump on a OM366LA

Hi again, sorry for all the questions about my engine but the air pump is making a bit of a squealing under load at idle (goes away when the pressure relief valve operates or RPM's pick up). At first I thought I needed the belts tightened but they seem to be pretty tight. Also I ran the truck for a few minutes after bleeding the diesel from the filter change and noticed the top portion of the air pump was already pretty warm. It looks like it may be lubricated/cooled by the engine oil? Should I just tighten the belts a bit more or is there something else I should be looking for?
Check all belts. I have chased that squeal on the compressor/power steering pair only to find tightening the other belt stopped the noise, even though the noise seemed to be related to compressor operation.