Hi Lucas,
You say you connected TXA to RXA directly via a cox cable? Did you put an attenuator inline? I am afraid you may have damaged your RX side by transmitting directly into it without protection. Most USRPs can be damaged with input power greater than -10 dBm. Disconnect that cable immediately and try testing the RX side alone to make sure that it is not damaged. Then, if you want to go back to a looback test, make sure that you put about 60 dB of attenuation between the RX and TX ports.
The noise you were seeing was almost certainly not a problem with your source block, but your RX being over-driven (dying).
Good luck.