Your receiver is too close to your transmitter and you are overloading it. It's the same thing with testing your voice signal with a local receiver and it doesn't sound right. Use a remote station like WebSDR to sample your PSK31 signal and consider turning down the audio input to your digital interface. Overloading the MIC input of your uBITX audio amplifier can result in a overmodulated signal like that too. If it looks fine on WebSDR then the audio input volume is fine, but full blast is usually too much.
