Sickly E46 318i

Our E46 318iA touring (M43 engine) completed its journey to the shops on
Saturday on the back of a recovery truck.

With cruise set to 70mph on the motorway, it kicked down to 3rd for no
apparent reason. Manually putting it in 4th worked and when I returned it to
automatic mode, it was happy to stay in 4th.

A couple of miles later on and still with cruise set, it kicked down again
and then lost power, slowly coming to a halt on the hard shoulder. The
engine still starts and sounds OK when idling. It was able to drive the car
the 20 yards from the back of the truck into my driveway.

Increase the revs a bit in neutral and there is a speed related
plastic-rapidly-striking-plastic type noise from the unit directly behind
the dipstick [from looking at the ETK, I think this is intake "adjustment
unit" 11 61 1 438 404]. I'm wondering if one of the vanes on
propeller-looking element of this unit has sheared, causing a restriction in
the air flow.

Sound plausible? Any other likely causes?

PS If you're ever stuck thinking of what to do with 8- and 5-year old boys,
arrange for your car to break down so they get a ride in a recovery truck -
apparently it's the best fun they've had in weeks...