Who knows what this could be?

Lately the ole XJ has been giving me a growling noise. It only happens when quickly accelerating from a stop. It only lasts a couple of seconds and does not do it every time. I have come to the conclusion that it is in the front end and probably a drivetrain component. THe odd thing is that it goes away so quickly. I have googled this and there are a few people who have had a similar problem but I never saw a reply that said what it was. PLease let me know if you have had a similar issue and what it was.
Thanks!

Check and or replace the oil in the rear differential.