Really loud humming noise coming from around air filter in engine compartment on 99 explorer when very hot outside or Air Cond is on but only when truck is stopped it seems to go when you accelerate?

The loud humming noise from the air filter area in your 1999 Explorer, particularly when it's hot outside or when the air conditioning is on, could be due to a few factors. It may be related to the air conditioning compressor working harder in high temperatures, or a problem with the electric cooling fan. Additionally, a worn or failing component, such as a belt or pulley, could also cause this noise. If the sound goes away when you accelerate, it may indicate that the engine's increased RPMs are alleviating the issue. It's advisable to have the vehicle inspected by a mechanic to diagnose the exact cause.