djp123

The engine check light only comes on if there is a drive by wire problem.( I call it a throttle light) it is common for the actuall accelarator pedal position sensor to play up. I would get the codes checked & I think you will find a accel. pedal sensor fault of some description & will require the accel. pedal assembly to be changed,sounds very remote I know but iv'e seen it a dozen times. Also that cap on the throttle body does blow off sometimes as boost pressure blows out between the throttle plate shaft & pops it off,this shouldnt worry anything as the engine is map sensed not air flow meter.

Springy motors are down that way (Springvale nth) & have all the ford flash datalogging software & are very familiar with that sort of stuff.They are capa dealers & are very easy to approach & very helpfull.

Sounds to me like you have some sort of issue with the throttle body (poss. dirty) & the noise you are hearing is the drive belt tensioner whipping back & forth when the idle drops. Does the engine check light come on at all?