Banbuk77

Today while standing on a red light (transmission in Drive mode) I slightly depressed the brake pedal (so it became maybe 80% pressed so to speak) and there was a nasty grinding noise coming from the transmission.
Like the gears came into contact, but didn't fully engage.

The car drives fine and when taking the foot from the brake pedal - the car starts moving without any problems or noises.
I thought the gears are either fully engaged or are fully apart and there shouldn't be a situation that they are making a slight contact.
Should I be worrying that something is not working properly?
Also, could the transmission get damaged because the gears were hitting one another for a second or two?

Non

I think grinding noises are definitely not normal.

I had a similar problem years ago on my Acura, and I solved it by replacing the fluid. My Acura had 150,000 miles on it at the time so I did not do the flush, I replaced the 3 quarts of fluid with a good quality synthetic to mix in some "fresh blood" and the occasional gear grinding stopped.

So if I was in your shoes I would start by replacing some fluid.

How many miles on your car?

Clutchless

How many miles and what year is your RX? You should probably get the transmission fluid changed and if all wheel drive also change the transfer case fluid. Also check your motor mounts and transmission mounts etc as maybe it was something external creating the noise. Worn mounts can allow them to move and create lots of unexpected noises and vibrations. Finally, check your brakes.

Banbuk77

I don't think the brakes were making it. If they were making this noise - that would mean the car was moving, but it was not. Just the noise. That's why I thought those were the gears.
The car is 2013, 60K and in a good shape AFAIK. I bought it Certified a year ago and still have 1 year of warranty left.
I hope there is nothing wrong with the transmission since you are not supposed to put less pressure on the brake pedal. when idling. You either press it fully or let your food off it.

Clutchless

If you press very hard on the brake pedal you activate the hill holder feature and may see a light appear on the dash. The car may then hesitate a bit as the brakes release when you let your foot off the pedal.
60,000 miles is a good point to get the transmission fluid changed. Call around to both Lexus and Toyota dealers for price and because some dealers offer this service and others will say you should never change the fluid, which is both wrong and short sighted. The owners manual states that under towing conditions it can be changed at 60,000. It should state that everyone should change it at that point if you plan to keep the car past the warranty.