I used a home brew version of the DL4YHF counter, and also had noise.? To get rid of it, I installed a 100uF cap. across the power connector at the counter.? Put a 10mH RF choke in series with the positive voltage lead. and installed another 100uF capacitor to ground right at the power switch. Don't know if you could use other values of RF choke, the 10mH was what I had on hand.
DuWayne KV4QB
DuWayne Schmidlkofer