Thank you both. It is indeed the rupture disc. I have never had a disc failure before on anything...gun, tank, or compressor. Probably has to do with the fact that I never overfill my guns, I shoot regulated/tethered when on the bench, and most of all I'm able to shoot at home. No need to do 4500 psi fills on tanks when 3800 to 4000 (the highest I'd gone until recently) more than gets it done. If I run low I can always just go to the garage and go from 2800 to 3800 pretty quick.
I've found that my compressor works VERY hard above 4000, but fills up to 3800-3900 fairly easily and timely. Even shooting a Texan .308 I can get a lot of shots from a sub-4000 tank fill on 74 cu. ft. tank. However, I now have a Texan .50 and I was concerned that my bench time would be reduced since I wanted to be able to stay around 3600 if I chose to on the gun. I set the compressor to around 4100 and just about the time it was about to shut off, BAM! I've got 20 hours on the unit and it's about 4 years old.
So a simple fix, I hope. And I'm now loathe to run my equipment that high. I know I 'should' be able to, but it doesn't seem worth the risk/wear and tear on the equipment. It seems everyone but me has disc failures, blown fittings, compressor failures (mostly Yong Hengs), and it always seems to revolve around getting that last couple of hundred psi.
Thanks again for steering me and I'm just sharing some of my usage data in the event that it may help someone else. Cheers!