Have a strange sound I need help diagnosing

[Vector]

When I accelerate from a stop with my 2007 Sub everything sounds and feels normal. However if I get on it from a dead stop, I hear what sounds like a small thud. It only happens for a second, and not again until I stop and get on it again.

I was thinking maybe a failing U joint. I don't think it is a motor mount because I do not feel any shifting of weight. I've never had a transmission mount go bad, so I have no clue if that could be it.

Any ideas?

 

[YukonButICanToo]

Sounds like a slip in the driveshaft. Try greasing the slip joint first.

---------- Post added at 11:44 AM ---------- Previous post was at 11:37 AM ----------

Does it sounds like this

http://www.youtube.com/watch?v=GFC8boB7RRs

 

[Vector]

I was hoping it was only a u joint, but unfortunately I have confirmed it is a motor mount.

Anyone now a good vendor to buy them from? I'd like to avoid paying dealer prices if I can help it. Also how big a job labor wise is it? I have done a lot of repairs over the years, but never replaced any mounts.

TIA