Have the same issue with my INT-30, figured there was something defective with the amp as I bought 2nd hand, although SQ not affected beyond the momentary interruption from the noise itself. What stumped me is that it would happen even when the unit is unplugged from power source. My observations:
1. The sound is a sharp metallic "ping" or "pop" - sounds like metal contacting metal.
2. It can occur even when no power to unit, so must be mechanical not electrical.
3. Ping occurs 1-3 times over 5-10 mins when amp is turned on, but does not occur when fully warmed up. Same sound at similar intervals when unit is powered off and cooling down.
Conclusion: The sound is the result of the expansion and contraction of the external heat sinks bolted to the amp chassis, which is why it's heard only during warm up and cool down, but not after running for 15 mins or more. At least that's my best guess given the above symptoms. Since the amp sounds great otherwise I've decided to view it as a lovable quirk and instead of an exceedingly minor annoyance.
1. The sound is a sharp metallic "ping" or "pop" - sounds like metal contacting metal.
2. It can occur even when no power to unit, so must be mechanical not electrical.
3. Ping occurs 1-3 times over 5-10 mins when amp is turned on, but does not occur when fully warmed up. Same sound at similar intervals when unit is powered off and cooling down.
Conclusion: The sound is the result of the expansion and contraction of the external heat sinks bolted to the amp chassis, which is why it's heard only during warm up and cool down, but not after running for 15 mins or more. At least that's my best guess given the above symptoms. Since the amp sounds great otherwise I've decided to view it as a lovable quirk and instead of an exceedingly minor annoyance.