Clicking sound from back

In my last thread, several people recommended using nuts/soldering/marine connectors to connect three pairs of wires since the connector was chewed up by careless disconnecting. I used three new wires to connect each pair. The car now starts and is ready to go into D or R. However, the red triangle still comes on. I also hear a couple clicks from the back when starting up.
Now when I've been driving between 1/2 to 1 hour, I hear one click from the back. With that, the car won't accelerate quickly, and the hybrid battery level on the screen won't fill up, it stays low and gets down to one or two bars... I'm stuck initially at 15-25 mph until it eventually gets up to highway speeds. This makes stop lights/signs a hazard.
I'm due for an oil change at 183k, but I have noticed that the top black air vent at the hybrid battery is leaking air (room temperature at least, not hot air). Is this because I've put the vent back on wrong? I've attached some pictures.
Seems like stuffing rags around the top vent's entrance to the battery, where air is leaking, helps... the battery level is able to stay middle or high. Has my battery been overheating bc it's not getting enough air?

 

Under what metal?