The amplifier module heat sinks are aluminum, and the chassis is steel, which means that they heat and cool at slightly different rates. Sometimes this will result in a faint sort of occasional clicking or ticking noise. If that's what you're hearing, then it's nothing to worry about.
A click or 'ping' is as keith says, normal for heatsinks.
A possible manufacturing upgrade would be to insulate or use some heatsink compound between the aluminum and steel parts.
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