Any ideas what this could be?

[broadside]

I've got a nice TD5 Discovery 2 with 115k on the clock. There is a rumbling noise coming from the front of the vehicle which sounds like a bearing. However, the rumbling noise disappears when steering left, returns to "noticeable" when driving straight on but becomes noisier on steering right.
Any ideas where I should start to remedy this or what it could be?

Thanks.

Broadside

[broadside]

Thanks for the replies. Very informative and useful. I'll get it booked in for a new hub.