Long piece of rubber in radiator?

Hey all!
I was flushing my cooling system with distilled water multiple times until it ran clear. On my last cycle I noticed my upper radiator hose twitching and hitting the air box which caused audible thuds. After witnessing this I shut the vehicle off. I proceeded to squeeze the radiator hose to clear potential air bubbles and this piece of rubber floated the the top of radiator port.
I pulled the rubber out and started the truck again with no issues and the heater blew hot again. Should I be concerned with this piece of chewed up rubber? Only thing I can think of is potential RTV that was used for water pump? I plan to replace radiator, water pump, and hoses soon which is why I was doing a flush.

 

Looks like a manufactured piece. Thermostat gasket?

 

Thank you! Didn’t think of that.. possibly from old one.. I’ll take it apart soon and hopefully I find where it came from. My immediate thought was head gasket but that’s metal. My heart sank for a moment lol

 

I found something similar in my heater hose when I changed them.
Was a piece of RTV
Never noticed any issues from it or after.