Just bought an Orange TH30 and have only started having this problem after two or three weeks of owning it. So basically after warming it up for a bit i flick the standby switch to full power. Then i hear this strange noise for a good 5 seconds - it sounds almost like the waves at a beach but more aggressive and harsh. Has anyone heard this sound from their amps and is it something i should be worried about?
Hey man, that REALLY sucks... But if anything I bet those tubes have been in that amp a while (especially if you bought it second hand or from a music store that lets people play them) so first thing I would try is to just change the tubes, if that doesn't help then id take it back.
There is no way an amp should start making strange noises in the first few weeks of owning it.

oohh yeah and save the old tubes, just incase new tubes don't help, put the old tubes back in
I would actually go in the store and say "I just bought it and I hope this isn't the kind of quality your shop wants to share; fix it."

Ive just tried to isolate the problem so i disconnected anything going into it including the fx loop, channel switch, front input. Now i didn't get the sound anymore hmm. Ill try it again to see if i can record some audio if the sounds comes back again.