A faint transformer hum you can hear only with your ear near the unit is normal for anything with a large power supply. If you have to hunt for it, it is not a problem.
The physics is boring and reassuring: mains at 50 Hz makes the transformer laminations vibrate slightly, and because the vibration follows both halves of each cycle, the hum sits at about 100 Hz. A big AVR has a big transformer. Every serious amplifier does this to some degree. What separates normal from worth investigating is threshold and change.
Worry thresholds: audible from the seat with the room quiet; a hum that has changed recently, louder or different in character; or one that varies with the time of day.
That last one matters more in India than in most places. Mains DC offset and voltage distortion make transformers buzz louder, and the usual sources are heavy appliances, inverters on the same circuit and certain dimmers. This is why the hum can appear only in the evening, or only when the air conditioner compressor kicks in, and disappear at 2 am. The AVR is fine; the electricity arriving at it is not.
Diagnostics that cost nothing. Try the AVR on a different circuit, ideally one that does not share with the AC or the inverter. Note whether the buzz tracks specific household loads switching on and off. And separate it from hum coming through the speakers, which is a ground-loop matter and a completely different problem; our hum answers cover that one.
Sudden loud buzz that persists on a clean circuit and does not track any appliance? Authorised service, and mention exactly when it started. Faint constant hum that has always been there? Close the cabinet door and get back to the film. The transformer has been humming since the day you bought it; you have simply had a quiet evening.
If the buzz coincides with an inverter or dimmer and you cannot move circuits, we can talk about mains conditioning for the AV circuit, but I would do the free diagnostics first.
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