2001 Ford F-150 4.6; idle problem
 

I'm having a strange problem with my 4.6 F-150. The idle is rough and has a rumble to it. The RPM's do not change while this occurs. I have tuned up the engine entirely, and ran a scan on the motor with a detailed program. It checked virtually every variable on the motor, and nothing turned up to be noticeably wrong. Above idle, throttle response is fine and there are no mis-fires. A friend of mine suggested that it may be the ethanol 87 gas, so I ran down the tank and put in 93. This gave me no noticeable change. Can anyone provide some insight?
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