Does your TM have a built in transducer? If so, are you using it, or one attached externally?
You changed NOTHING but the battery? No new wiring attachments or moved any wiring from one battery position to another? Going to AGM isn't going to cause interference - something else is doing it. Have you moved around any wiring in the boat? Tied any of the wiring into tight bundles? The interference is only when TM is running? Any speed, or only higher speed?