Not sure if mine was the same as yours but after I took my car in for the 5k maintenance, I noticed a noise coming from the engine similar to what you have described. The car still ran well but I knew it didn't sound the same as before. I expected the service department to say it was "normal", and they did, but the driver knows the difference! My Lexus rep was great and took a look at my car to see where the noise was coming from and after I got the car back, it sounded pretty much normal, i.e. brand new normal. I was happy. My Lexus rep was happy too although the mechanic said he "didn't do anything" to fix it. He just checked and ran diagnostics on it. I sort of knew there was no problem with the car, just the settings needed to be adjusted, and I think that was the case.
I'm glad I went back with the noise concern and was fortunate that the service department looked into it instead of saying it was just in my head :-)
Good luck!