Will be sending this unit back

We bought this device to take on a tour in the UK. Five days into the tour, the device is charging our phones, powering a laptop, charging electric toothbrushes, etc. The problem is, after it runs for about 5 minutes, it starts emitting a high-pitched squeal, or an electronic growling sound, or it will alternate between the two. Turn off the power to the unit, count to five, plug it back in - and the noise stops. But it always comes back. Not okay when you're trying to sleep.

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