Long time referencer, but first time poster. Stock lt1 with suspension upgrade.

Ive noticed a rpm change and rumble thats not the usual. Only today has it revved at +2000k for few seconds at start up. Gut gargling sound. It can start up fine then i can start it up and it will have a noise. I presume its in the exhaust system, maybe muffler? but why the high rev? Video compares non rumble start up with a rumble noise start up. Rpm thing was random and hasn't happened prior to today. Suggestions would be awesome.

View My Video

 

How many miles on the car? Has any exhaust component ever been replaced? Could be a failing cat causing the noise once its warm

 

I've only noticed it doing it once the car has set for a bit. Block still warm, but the cat hasn't warmed up quite yet. 169,xxx miles and nothing modded besides normal maintenance (wires, filters, grommets, etc...) and suspension upgraded.