Strange tachometer behavior

[mark_tyler]

Hello!

Got my Mazda 3 two months ago. Have recently noticed a strange behavior of tachometer. I happens only while the car is stopping and when
driving in neutral or with clutch down. When it stops, the rpm is fine.

In the video after speed up to 2000rpm the gas pedal is released.

I had some engine repair before but i can't tell whether it started after repair or it was before (i had my valve seals replaced).

 

[Haxir]

looks like the ecu is constantly trying to figure out what the rpms should be.

 

[meicalnissyen]

might be a small air leak between the maf and manifold