td5 vibration and rumbling

Yesterday I washed my td5 at petrol station and set off home.
Upon the approach for a roundabout I downshifted into 3rd,a loud and very noticeable vibration came from the front end.Only the once.
Later that evening,another roundabout,3rd gear again same thing,this time I dumped the clutch as I thought my box was gonna nip up,it went away.
Today iv had the rear offside wheel off for an unrelated brake cleanup (as the whole offside has been binding) I noticed the fuel filter housing is slightly weeping around the seal and at one of the unions,so the possibility of air in the system occurred to me...
Could it have drawn a little bit of air into the system and that caused it to "misfire" terribly then clear itself??
Almost felt like it lost power briefly too?
Or is it likely to be the box???
Box normally gets all gears ok,reverse is sometimes a pain when cold but I have an Astra that's the same.
Clutch is ok,no noises from either things.
Checked the box oil level and that's fine.
Couldn't check trans box oil as I couldn't get at the fill point.
Props seem fine with no play too.
Any help,advice or suggestions are allways appreciated.

one thing i can say: the fueling system is under pressure, it will leak out fuel not let air in ... your problem seems to be transmission related and that's not something i can help, sorry

Hopefully this won't be the right answer, but... I had something very similar last week. Just a few occassions when I thought an engine mount had failed, the vibration was that bad. I also started to get some nasty 'graunchy' feeling through the clutch pedal, but otherwise all was fine. I checked underneath to see if anything was loose or worn, including the prop UJs, all good. I decided that as the clutch still didn't feel right that it may be the spigot bearing or maybe the clutch release bearing that had/was failing, so I bought a new clutch, dual mass flywheel, bearings etc...

I was kinda right - it was the DMF that had failed - it was falling apart. Now that the new clutch etc has been fitted, the gearchange is smoother, lighter, and a vibration that I hadn't really noticed has gone. It cost me over £700 to do, so as I originally said, I hope this is the wrong answer, but it is something worth thinking about.

If you haven't changed your prop UJs in the last year or so, then its def worth checking those for wear - if they fail, well the consequences don't bear thinking about.

+1^^^ reading above i realise that i can remember some shuddering on downshift before my DMF went...good point

Thanks for all the replies.
Some great input.
What is the best way to check propshafts for wear please?
Articfox-was that supplied and fitted I take it??
Its a strange one as I have experience of driving cars with failed boxes and dmfs as I used to work at a clutch and gearbox centre.
Iv just done 6miles in it and all was fine just now.
Suppose its going to be one of those where when it goes its gonna go big time lol

Still seems ok.and iv done 60miles........