With the DI units there are transducers specifically designed for the trolling motor. They have more internal shielding and can help prevent interference.
The transducer that comes with the unit is an XNT DI 25 T transducer and is intended to be mounted on the transom .... a mount can be used to mount it on the trolling motor, but without the shielding is more susceptible to noise than the XTM model transducer.
There are 2 versions of the XTM transducer one is the XTM DI 25 T and the other is the XTM 9 wide Di 20 T .... I'm not sure if the Helix 7 supports the DI wide or not.
Did you exchange the transducer that came with the unit with the XTM style transducer?
If you don't have the XTM transducer it would be cheaper to get it than to swap out the trolling motor. There is no guarantee that it will eliminate all the interference, but in several cases I have seen it do so.