Problem with Garmin echoMAP CHIRP 75cv fish finder

Hi folks, i'm having issues with my Garmin CHIRP 75 CV fish finder and I was hoping someone would be kind enough to point me in the right direction. The transducer is the one that came with the unit and is transom mounted. When I am slow trolling with my 40 hp engine I am able to see defined arches signalling fish on the screen but when I come to a stand still (anchor) or when I am drifting I am getting a lot of what I would describe as "noise" (crazy wavy lines and other lines that do not appear to be weeds or fish in the water column) - see attached photos..... Has anyone else had this experience or know what may be the problem?

Many Thanks!