In message , Bill Wright
writes

My nephew is an airline pilot. He says if it was a four engined turbo
prop it would be military.

Thanks Bill. That certainly makes sense. We are about 45 miles from
the nearest airport (Aberdeen), but very rarely see commercial flights.
On the occasions they do fly this way, they are much higher, and almost
silent, whereas the two yesterday were relatively low, and audible.

--
Graeme