I wondered about that, there are no other noises however.

In the past I have only seen DMF issues when they are already Falling-Apart bad so don't have any experience of early failure modes.

The absence of any smoke (Which I would normally expect from a misfiring diesel) would also point towards a more mechanical origin.

(Though I can imagine that ingested water and a bent rod might produce a similar effect. But, as I said in OP, I really don't think this has happened)