I assume you mean that the revs are over 2k, if they were over 20k I think you'd get a different sort of noise from the engine bay.
From the description it sounds like you'd got a damaged syncro, especially if this happens when you're shifting down into 3rd. Up shouldn't make quite that bad a noise but if the syncro is really on the way out (which your description suggests it is, given that it's been getting worse over time) you'll get the noise on upshifts, too.
Pretty much the only way to fix it is to pull the gearbox and either replace it with an overhauled one or strip it and replace the syncros. You can also hope that you haven't damaged/worn any gears themselves, that'll make the repair even more expensive. As you can probably guess already, that's not going to be a cheap repair.
In the meantime I would suggest that you double declutch and rev match on downshifts (ie clutch -> box into neutral -> blip throttle -> clutch and move into lower gear) and use slower upshifts to extend the life of the gearbox until you can get it fixed. There's no point in trying to change gears as fast as possible (even on a race track, the gain is minimal) but it's very hard on the box.