Sorry about the misinformation on the lifters I finely found them in the manual and they are self adjusting hydraulic. Great post USMC thanks. With that said it probably a little lifter bleed off like EV2DEMON has stated.
All my old Chevy small blocks had some degree of valve train clatter on start up once they had some miles on them. As long as it isn’t crazy loud and goes away shortly after the oil pressure builds I wouldn’t worry too much.