Originally Posted by yshakhov

Be smarter then they are. Go to radioshak to buy digital audio recorder and then install it in deferent places. Make an recording and figure out where this noise come from.
I do have more complicated noise.
“Mazda3 2008 2.3L (AT)
When I drive a car on quite route at 35-37mph at 5 gear speed and slightly accelerate, RPM drops from 1750rpm to 1500rpm as a result of it I hearing VALVES slightly knocking.
It doesn't happen when you switch manual gear level.
I was in dealership two times and they didn't hear anything or doesn't wont to hear.
What kind of sensors is malfunction?
What do you think?”
Today I got OBDCOM diagnostic tool, digital recorder. I'm going to collect all evidences and then go to dealership.
I think they doesn't want to hear anything under a warranty until you show them what exactly problem is and show them how to fix it.