Intermittent Grinding Sensation In Accelerator When Cruising

[Anska]

Help please. (I posted this in General but it's particularly my Mondeo at issue so reposted here - hope that's okay and doesn't annoy anyone)

I feel an intermittent grinding sensation through the accelerator pedal and occasionally the clutch pedal when cruising.

Passengers can't hear it so I'm assuming I'm imagining that I can hear it.

What could it be?

For example, it often happens when cruising at 30 miles an hour in
3rd or 4th and foot resting gently or feathering accelerator to keep at
speed. However, it can happen on the motor way cruising at 60-70 as
well. The main thing, it happens only when cruising not breaking or
accelerating/pulling away. Depressing the clutch doesn't stop it.

I've just had new clutch and DMF but this problem was from before -
I'm noticing it more now, I think. I've also just had full electrics
diagnostics which show up no major problems (cruise control occasionally
turns off when I press SET- (minus) but I don't really care about that
too much). No warning lights showing. 2 mechanics have done short test
drives and think it's a smooth driving car and can't feel the grinding
sensation.

I have Ford Mondeo Zetec (2010) diesel, 140bhp, 1997, 35,000miles ish.

Suggestions please, folks!

Thanks for any help. Anska.