knocking on warm start

[MPG]

Is the following a discription of timing chain tensioner or something to do with the hydraulic tappets? When the engine is cold it starts fine and is quiet.If I stop for a couple of minutes it starts fine again. However if I stop for more than 15 minutes then restart again it fires up fine but sounds like a diesel ( well not quite.) Blipping the throttle it goes but when back to idle its still there for a couple of minutes then fine again. Its a xjr v8 98 with 57000 on it.

[triple7]

Where's Groomi when you need him???