only when the engine is cold. It's like a shriek/rattle as one accelerates away, probably at about 25mph and lasts for about 2-3 secs at most. Sounds to me like something that's oil starved and partially siezing, I'm wondering the belt tensioner or pulleys? The pump and belt were replaced about 1000 miles ago. Stragely it's only started since I had the box oil changed (spoken to workshop and they are puzzled). I replaced the 2ndary tensioners the day before the oil was done and didn't notice anything afterwards so don't think it's the engine.

 

You may have hit the nail on the head with the belt tensioner. In 120,000 miles, my XK has consumed 3 of them. First time I changed the whole assembly, subsequent service, just the pulleys. Someone on the forum once used the term the "Famous Fighting Gerbils" to describe the sound. Never forgot the term and giggle when I think of it. Good luck.