I replaced the thermostat in my 93 RX7, (which required disassembling nearly 1/4 of the engine bay). When I put it all back together, I am left with a hose that I don't know where it originally went.

The hose is the one that comes off the right hand side of the air filter assembly. It has a ] shape. It appears that it would connect to something under the air filter?

The car seems to run fine without it, but I get a whistle when running the car. kind of annoying, and it has never done that before...

--Edit
I should add that I did remove the entire air filter assembly. I tried to leave all the hoses hanging, but apparently this one came fully off.
I do own a matinee manual, but can't find the air-filter section (if it even has one)

 

If it's this, it's the hose to nowhere (does not connect to anything).



I'm not sure it's purpose though. Maybe if the filter gets overly clogged, you have some airflow.
.

 

That would be the one. Cool, so I am not insane! Now the question is if thats factory how did I introduce a whistle? (should I create a new thread for this?)

 

It shouldn't whistle. Try plugging it to see if you still have a whistle. If you do, it's something else

 

Remember...in stock configuration the blow-off valve empties into the stock air box. That's the vent hose for it. But you're right, it just hangs underneath the box.
Generally a whistle is a coupler leaking somewhere on the intake track under boost.

 

This is a "Tube to nowhere" in that there is no connection. It is a just vent tube and doesn't connect, just hangs there. Drove me crazy trying to figure it out originally...