I have a few lifters making noise until well warmed up

Strange no noise today . Can't see it being sludge , I change the oil and filter often and have used synthetic oil from 5,000 miles on . Tested last night and could not find an exhaust leak . Very strange sometimes it makes noise , like yesterday ,started in the morning noise after letting it sit for 3 hours went to start and there was the noise again , today started up no noise , let it sit during the day started no noise again . Very strange , either it's a lifter with a sometimes stuck plunger or a very odd exhaust gasket leak .
I will track it down ! Thanks for all the ideas on what to look for and possible solutions .