Location: Belgium.
Time: yesterday noon.

Normally, when a jet fighter (F16 99.9% of the time) passes over, they fly at about 300m (1000ft) altitude, at their typical high subsonic cruise speed. Quite noisy, but soundwise they come and go in like 10 seconds.

Yesterday, something with jet engine passed over. It sounded loud and deep, far louder than passenger planes at low passes. If it would have been a passenger plane (which it wasn't, see further) I remarked that it sounded like an old Russian beast. It was very cloudy and we couldn't see what it was, but my brother could take a glimpse through a hole in the clouds and he identified it as a jet fighter. He's not the ultimate expert on aircraft, but he has visited multiple airshows so he knows a jet fighterish thing when he sees one.

The thing is, the sound lasted for at least a minute. And it died out very slowly. The sound never diminished to increase again so I assume it was a single aircraft. F16's here usually fly alone, sometimes with two or three but then you clearly hear one die out before the next one is getting loud.

Assuming it was a single fighter-like plane, is their anything that looks like a jet fighter but typically cruises a lot slower? I don't know, some small jet bomber like a F111? Or could it be that, because of the low clouds, it was an F16 flying higher than usually and that caused the sound to be audible much longer than normally? It couldn't have flown very high as we wouldn't have been able to identify it as a jet fighter(ish). Would a difference between 300m and, say, 800m make such a difference in how long you can hear it?