If it's synchronous with pedaling and only happens on the large chainring, it could be the chain passing a shift gate and coming down a bit hard on the next tooth.
The giveaways here are:
1- the timing
2- only happens on large ring (the smaller one doesn't have shift gates)
3- varies, depending on which sprocket on the cassette with the most in-line ( 3-4th) sprocket being quieter than the ones to either side.
If you suspect this is the problem you can confirm it by coloring the tips of the chainring teeth with a black marker, riding a while then checking to see if the color is worn one or two teeth.
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