Strange sounds

This is Plymouth, SouthWest, UK
an hour ago around 13:10 17/01/2008 i heard a noise outside, sounded like the fighter plane flying, so i ran out on to a terrace and looked at the sky but no luck could see nothing but the sky, there were almost no clouds (only one at the horizon and thick mist at Plymouth Sound)
when the noise disappeared i went back to my room, but on my way back heard it again, so ran back to the terrace but still didn't see anything, i got the feeling that it is flying somewhere near cause the sound would not disappear... could do nothing but search through sky for any objects, but no good...

Helicopters aren't that rare in this area and you can periodically see planes, from Plymouth Airport, but this one, i've no idea .. the only thing i can connect it with is the fast moving plane on a pretty low altitude

does anyone have any ideas, i wouldn't say these were aliens cause the possibility of fighter aircraft or some other is still possible..
but i remember reading once a thread on flying objects which one can only observe in infrared light.. some guy made experiments filming with night vision... can this be the case ? i don't know

Today heard it again. Even though it is pretty cloudy, i'm sure it was not a sound of a helicopter.. Next time i hear it will try making some infrared photos, who knows, might help

You say it sounded like a fighter jet, so, that was probably it. They are fast...very fast, and can be pretty much out of site in seconds. Not saying that's 100% what it was, but it's the logical conclusion.

When they're really pushing it, a fighter jet can nearly fly in front of its sound wave. By the time you're hearing it, it is already well past your position.