Engine noise beween 1000 and 1200 RPM

It's the transmission still in a REALLY high gear with revs very very LOW while you are going slow. The TCU has not chosen the 'best' gear to be in, rather it has chosen the 'most-efficient.'

Common noise/vibration in fuel-efficient 4-cylinder cars.

To test it, put the GLA in manual mode. Coast along at about 20-25mph, but put it all the way into 6th or 7th and push on the gas, it should re-create that sound

Thanks. I believe you're right, but can anything be done about it because it's very annoying?It happens in speeds between 30 and 50 mph while transmission between 5 and 7. These speeds are very common in city traffic, so the noise happens quite regularly.

Use manual mode or put her in 'sport' mode after warm-up.

I tried, it doesn't really help. Nobody else has this problem?

Take a video, I'm very curious.

Is this your first turbo-4-cylinder? There are many familiar sounds for those attuned to this driving experience, so we might miss exactly what you're describing.