danfox

A little slack is not a problem until it jumps a tooth, then a big problem. A little like being pregnant or not. A belt squeal is not a good thing to have....one of them needs to be snugged up, but the timing belt will not squeal. If you hear he squeal, it is an accessory belt.

Did this start after the belt change? Did fuel mileage drop. Could have jumped a tooth on the timing belt due to not being tensioned high enough. Could also have a failing idler on the belt letting the belt slack a bit and jump and explaining the noise from the cover.

As a general rule, you will find that hybrids in SUVs are not designed for fuel mileage so much as performance. Although it is the same hybrid technology as the cars, it is applied to these heavier vehicles with difference strategy to punch-up acceleration performance. The engine comes on sooner. This is true of most brands, not just Toyota/Lexus. Check the published rating of a car w/ w/o hybrid vs SUV w/ w/o hybrid.

A close friend of mine has made his career in fuels development for BP and Amoco. He clearly states that for an engine where premium is recommended, you will get better MPG and performance with premium, but that most modern engines can run just fine on regular. So, you will get better slightly MPG and performance on premium but it will likely cost more per mile because you perform improvement is less than the 8-10% cost difference of the fuel. Use premium because you want the performance, not because you engine 'needs' it.

I agree with the comments on the evaporative control and purge system operation, but that cannot explain the fuel pump running. The pump deals only with the liquid fuel and cannot influence the tank vapors to reduce or purge. I have also experienced what sounds like the fuel pump running hours after my 2007 RX350 was last driven. The vehicles always runs fine. On two occasions, I have found the battery down to a level of 'no crank' for no apparent reason of lights or accessories left on. Could it be the pump coming on? It has always shut off when I noticed it running, but I haven't stood there and waited for it. Anyone else experienced battery drain for no reason when they have also heard the sound?