I would confirm that you have "no exec" configured on each of the lines. Running the exec process on the line sometimes leads to the router interpreting line noise as characters ... ie an attempt to login. This creates more "noise" on the line.
If it's not the exec process, another cause could be a bad cable.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...