Avalon Hybrid Intake Leak

Those symptoms definitely point to an intake leak. Increased fuel consumption happens when the engine tries to compensate for extra air. The hissing is the air being drawn in. Have you checked the intake hoses, especially the corrugated parts and where they connect to the intake manifold? A cracked or loose hose is likely, since the rubber might be degrading. Are you sure it's not the turbocharger?

I've glanced at the hoses, but it's hard to see well. The 2014 Avalon Hybrid doesn't have a turbocharger. Could the intake manifold gasket be leaking, or are there other parts I should check?

You're right, sorry, the Avalon Hybrid doesn't have a turbocharger. I was thinking of another Toyota. A leak in the intake manifold gasket is possible, but less likely than hose issues. Because it needs careful diagnosis, and you could damage something, it's best to see a mechanic. They can use tools like a smoke tester to find the leak.

Thanks for the advice. I'll book an appointment with a mechanic. The repair was replacing the leaking intake hose, costing 100€.