Hello folks !

This is my second post on this forum, so please excuse my lack of experience.
I bought a Volvo V40 1.6 16V (B4164S2 engine), the 2000 model with manual transmission, and I'm extremely satisfied with it... BUT...
While idling, there is a small movement of the rpm meter arund 750rpm (600-800) from time to time - about 2 to 3 times a minute. Initially I thought it was because of the idle motor / idle control valve, but it seems like the problem persists even when holding the pedal so the engine is at around 2000rpm... the rpm indicator still moves from 1950 to 2050 (approx). The tests were made with engine warm, no AC on, no lights on, in neutral.
I've changed the air, oil and pollen filter and also changed the oil with Castrol GTX3 Magnatec 10W40, Purolator air filter and DENSO spark plugs (Volvo compatible, taken from a compatibility catalogue). The problem didn't dissapear. Otherwise there are no warning lights on the dashboard, the car doesn't hesitate when accelerating, everything works fine.
Other problems - maybe I'm paranoid, but when decelerating (downshifting from 4th to 3rd gear, let's say at about 30mph), there is a whining sound coming from the gearbox... is that normal when "engine breaking" (using engine to reduce speed) ?

Thank you !!!