Xbee receiver producing garbage characters when not transferring data
I had a similar problem recently with an XBee device receiving an unexpected sequence of six characters while not receiving anything while in transparent mode. It's not too easy to track this down because the diagnostic "pitch mode" isn't mentioned in the datasheet but after discussions with Digi support once I was aware of the name I tracked down this knowledge base article:

http://www.digi.com/support/kbase/kbaseresultdetl?id=3325

I was using a Sparkfun XBee Explorer Regulated and as mentioned the RSSI line was connected to a LED to ground pulling the line low that caused it to enter that mode. I removed the current limit resistor R4 connected in series with pin 6 and it solved the problem. But of course if you're using a different product the reference designators will be different so check the schematics for the particular board you're using.