Volkswagen Repair: 2005 Passat TDI putter or backfire after oil change, oil filler cap, oil drain plug

Question
I just got my oil changed on tuesday and driving home I started hearing a putter and I thought it was a flat tire or something stuck to the tire but it stopped.. I rolled down the window to listen and it started again.. sounds like it is coming from the engine compartment and happens when I am doing around 40-45 and let off the gas.  my question is what could they have possibly done while changing my oil that would cause this. They put synthetic in it, which I thought was a good thing.

Answer
The only thing I can think of is forgetting to put the oil filler cap back on, which will cause loss of vacuum in the engine crankcase and consequently affect the engine performance.  There is a vacuum because crankcase vapors are supposed to be recycled for emission control purposes.  Oil changers are not known for being rocket scientists.  A friend of mine blew the engine in his BMW after the oil changer left the oil drain plug off!
 On the other hand this may be just coincidental, the problems vs. the oil change.  If you are getting a warning light, such as Check Engine, I'd jump on that with an OBD tester.