Wife and I drove our 1996 Explorer XLT 4x4 4.0L Automatic to phoenix the other day; it was 109 degrees there and drove back that evening 75 miles one way still 100 degrees until got back to our rural area at 90 degrees there. Noticed a humming sound coming from under the hood, thought it was fuel pump at first but it was coming from air filter/air intake tube area that goes to manifold. It ran fine and never overheated but I still don't know the reason it was doing this. The noise was gone the next morning when I cranked it up cold and hasn't come back on. Car is in excellent mechanical condition with new air filter, fuel filter, oil and filter, tune up, alternator, Has 182700 mostly hwy miles. Any ideas?

 

Was the humming sound constant or did it change with engine speed? If it changed with engine speed it could have been one of the pulleys or it could also have been the fan clutch having actually engaged the fan since it was a warm day. It's difficult to pinpoint sounds like that under the hood since there are so many surfaces for the sounds to reflect off of.

-Rod

 

The sound was the same at idle or with rpm's up.

 

Probably an IAC valve that's starting to get gummed up or worn out.

 

I will check it out for sure; even though the IAC is over on the intake manifold and the loud humming was definitely coming from the air cleaner canister area. Thanks

 

There might be a secondary air pump or something down in that area, but I'll caution again that sounds in spaces like an engine compartment can easily trick a person in to thinking they are coming from one location when they are actually coming from somewhere entirely different....

-Rod