Thoughts on brake chatter

I lost a brake line on my pickup truck,had it replaced a couple days ago. Occasionally, when i apply the brakes i get a brake chatter.
It has occurred 3-4 times in the last 4-5 days. No pattern to it happening.

Thoughts?

Disc, shoes? You think maybe you got brake fluid sprayed on the friction surfaces?

caliper bolt not tightened good enough?

I think @Bing is probably on track. Something got on the friction surfaces and they are slipping. OP can confirm this by noting if the problem gradually goes away. If it fails to improve or gets markedly worse, time for the shop again.

Thanks to all who replied. I think ‘BING’ is correct. I drove the truck around all day yesterday and NO chatter.