What are the signs of spark knock?

[MOTORCITYSTANGS]

I'm useing NGK plugs with GT-40 aluminum heads, timing at 14* and if I cruse under 2000rpm it's smooth , but if I get on it and it reaches 2000rpm I get a bad vibration till I pass, above 2000rmp it fine

 

[Michael Yount]

Spark knock/detonation/pinging usually isn't described as a vibration - the most common description is a sound like marbles rattling around in a tin can.