Engine intake air flow not smooth on certain speed and gear?

[ammophobia]

Sometimes when I run the car on 3rd gear at around 40 kph (25-30 mph), then apply around 1/3 throttle on the pedal to let the car accelerate up to 60 kph (around 40 mph), I can hear that the car sucks air with some unsmooth noise... The engine runs fine however, the acceleration feels smooth, it's just the air not being sucked smoothly. It also happens sometimes on the 2nd gear and 4th, but I'm not sure about the speed/rpm range...

What can be causing this? Worn air filter? I live in a country with dirty air and heavy level of pollution, and I haven't changed my air filter for 30000 km (around 20000 miles), because factory service manual here suggests that the air filter is changed every 40000 km (around 25000-30000 miles). I do occasionally clean the filter though.

Is it okay to try to run the car for a while without the filter on to test whether the filter is causing this?

Thanks.