My set up is such that I have another sonar unit right beside my 797 with both transducers mounted on the transom fairly close to each other. (Second sonar is for running at speeds above 10 mph and is a temporary thing unit can mount thru hull transducer for 797) The first few times I was out kept seeing little dots appearing at regular interval. Did not make a difference at what speed I was running with the big motor. At first thought it was electrical interference coming from the big motor, but with change of speed making no difference thought that would rule out the big motor. Finally thought about the other sonar being on while using SI unit. The attached photo show the interference.
By turning off the other sonar unit the interference went away.