Very odd sound when applying accelerator.

[red eye]

I seem to remember having something similar on my Focus years ago which turned out to be a bearing in the aux belt tensioner.
Get someone to rev the engine gently while you stick your head under the bonnet and maybe you can isolate it.

With a long screwdriver, hold the handle to your ear and the pointy bit to the part. Mind the rotating bits you don't want the screwdriver to catch and take out your ear.
Any rumbling sound would be the bearing that has died