Resonant harmonics from bar tread patterns are common.
I think the K60s had a tendency to vibrate at lower speeds. Felt as a "coming and going" rhythmic vibration through the bars.
Especially under light brakes at lower speeds, which would lead me to suspect the front as the source. But it would be acting in concert with the pattern on the rear to generate the harmonic.
If you ever run Mitas E09s you'll notice the WW2 Jeep bar tread noise coming from the tyres. As they wear they start a harmonic thrumming, it's actually very loud, passing through a tunnel is a disturbing experience.