Interrupts hogging CPU on BeagleBone Black

[Sad Clouds <cryintothebluesky%gmail.com@localhost>]

I see a strange issue with NetBSD-9.0 on BeagleBone Black. When it is
powered via USB cable, everything is OK and CPU is normally idle.

But when it is powered via 5V DC cable, about 30-40% of CPU is
constantly consumed by interrupts (as reported by top).

Looking at output of "vmstat -iv" the only thing I notice is "irq 64"
which is a bit higher (34 vs 7) when powered via 5V DC:

5V DC:
$ vmstat -iv
interrupt           total rate
intc (cpu0) irq 98      0    0
intc (cpu0) irq 96      0    0
intc (cpu0) irq 32      0    0
intc (cpu0) irq 62      0    0
intc (cpu0) irq 70      0    0
intc (cpu0) irq 12   1723   10
intc (cpu0) irq 72      1    0
intc (cpu0) irq 30      0    0
intc (cpu0) irq 40      0    0
intc (cpu0) irq 41   1143    6
intc (cpu0) irq 42    884    5
intc (cpu0) irq 43      0    0
intc (cpu0) irq 64   5625   34
intc (cpu0) irq 28     41    0
intc (cpu0) irq 18   2143   12
intc (cpu0) irq 19      1    0
intc (cpu0) irq 69  16324   98
intc (cpu0) irq 3       0    0
Total               27885  169

Is there any other info I need to provide before opening a bug?