Who threw a set of marbles into my V8 XC90?
 

I have an 05 V8AWD XC90 that I have babied for 93K miles. This week I started hearing a deep tapping, rattling noise inside the engine. It is most noticeable when at idle and almost completely disappears when reving the engine in neutral or while driving. It truely does sound like there are steel ball bearings being thrown around inside the engine. No check engine lights have come on. Any possible ideas? Affraid of a $2-3000 repair bill but will have to find out come Monday.
Thank you.