It Humms because it doesn't know the words.
Seriously, it might be on its last leg. Check the fan and make sure it is still works. The noise may be the fan bearings wearing out.
You can buy a new 400 watt inverter for $27 at Amazon. This one has over 300 reviews and a 4 star rating.
Amazon.com: Cobra CPI 480 400-Watt 12-Volt DC to 120-Volt AC Power Inverter with 5-Volt USB Output: Car Electronics
06 Hurricane 34FT WH W20 Chassis 8.1L 120K, Steersafe, Koni Shocks, DIY Trac Bar, Tri-Metric 2025RV Battery Monitor, 4-6V Batteries, Scan Gauge 2, Crossfires, 735W Solar Morningstar MPPT-60, KVH R5 In-Motion Sat, XM Radio, 07 Chevy Malibu Maxx Toad, Falcon 2, Brake Buddy, Escapee