Yeah, could be - though it wouldn't be the brackets expanding it'd be the exhaust expanding & moving on the brackets.
You could try warming the bike up until the noise happens, then slightly loosen the exhaust mounts beneath the engine & at the rear, let the exhaust re-seat, then tighten them back up again..
Be careful - if they're corroded you might round the bolts, but if they're in good nick then it might be worth doing it anyway to give the threads a coat of copper slip.
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