Super low mileage, constant noise while driving

I have a 2006 with about 105,000 miles on it. In the last 6 months my mileage went from an average of 42MPG to 36MPG. There is a constant buzz/click/motor noise that happens about every 7-10 seconds. In April, I replaced my Hybrid battery (with only 101,000 miles) with a rebuilt one. But, the mileage drop happened before I replaced it.

I'm concerned about this noise and it's annoying. You're driving and you just hear it. Doesn't matter if the AC is on, if I'm in EV mode only, or what. You can almost set a watch to the timing of it. Doesn't matter how long I've been driving either, not an engine heating up issue in other words.

But I'm more concerned that my mileage is a pathetic 36.

I do mostly short drives, but even when I do longer freeway driving, it only gets maybe 38MPG. I used to get 56MPG when cruising at 75MPH for long distances.

I live in AZ, it's not cold EVER, so it's not a cold engine issue. It's FLAT here, like FLAT, so it's not a hill issue. I use Mobile 1 0W-30 Full Synthetic oil (which I've used for YEARS), I use fuel injector cleaners every 3 months or so. (usually Chevron w/Techron brand).

I feel like a pump or some sort of electronic is failing.

Any suggestions?

Thanks