this noise/humming sound is driving me insane, it suddenly comes on when the car is idle or even running as if the engine just kicked on, i can feel the vibration inside the cabin, is this normal? first time owning a hybrid and i dont expect such noise from a premium vehicle, almost at a point to trade in the vehicle, any suggestions are welcome

 

I had a similar experience, turned out to be a faulty water pump. Another engine noise issue that can happen is a stuck bypass valve in the muffler. The bypass valve is supposed to open only under full acceleration for more power, but if it sticks open the engine sounds much noisier at normal speeds. Neither of these are hybrid issues, can affect the RX350 equally.

 

So I got to see the dealer and he is saying this sound is normal for the hybrid. Guess, I want to get it checked from some other dealer as well

 

But if that would be the case, the sound should always be on. It only comes once in a while, stays on for couple of minutes and then goes away. At first, It did look like the radiator fan is running at full speed, which i checked by opening the lid and no, was not the case

 

Does the noise change with change in speed, power, or steering? A/C / front defogger on or off? A recording might help, if the issue happens frequently enough to capture.

I can think of no normal engine operating noises or vibrations that make the RX450h sound that different from the RX350 other than the obvious differences in how engine is started, and how RPMs are managed. There is some additional engine noise caused by engine loads that occur differently in a hybrid (RPMs may increase when hybrid battery charges, but not that much different than a conventional car's alternator charging). When cruising at moderate speeds with low power, the RPMs may be lowered almost to the point where the engine sounds like it is in a gear too high which may cause a faint rumble (something a car with a regular tranny would not normally attempt due to the delays of gear shifting), but it is a very efficient running mode that the vehicle will come out of with even a slight change of pressure on the gas pedal. But unless you are hyper sensitive, this wouldn't normally be that noticeable if at all to some people.

 

i will try to see if I can record the sound and post here, that may help

 

It could be you just need to become accustomed to the new noises made by your new to you hybrid. I am on my second hybrid and noticed with the first one that it made many strange mechanical noises which I attributed to the hybrid mechanisms. Same with the second one. I would bet money that you never do find any issues with your vehicle and that in a few weeks you will get used to these new sounds and learn to ignore them.
Having said all that, and given that the dealer said it is normal noise, you should drive at least one more RX450h to see if it makes the same noise.