Hello chaps

just a quick one:
I have a 2009 xt r with 750 miles on the clock,
I have only just recently noticed that when i start the bike up there is a rather loud vibration noise coming from somewhere in the engine (no idea where abouts).

It only happens when the bike is cold and being started for the first time,
e.g in the morning before work it will make the noise which will last approx 30 seconds. when i get to work and switch her off, i wait a few moments and try her again but she wont make the same noise.

Also it increases in loudness if i increase the throttle

Any ideas

thanks in advance

Andy