What noise is that?

First the most important questions...

Do you hear voices as well?

Edit * meaning from the same water heater?

How good are you with a screw driver? If you can take the top off, I can help you troubleshoot it..

there's a single screw at the bottom..

take a photo of the top right.. That's the pump motor section ..

Maybe also the bottom left which is the relay board..

When it's switched on, there are two led lights on the front..

The left one indicates elec power ..

the right led is activated via a magnet switch inside the pipe indicating water flow and switching on the pump motor .

The pump motor may just be loose and brushing against its cover which is the grinding noise you're hearing..

But I'm really just guessing.. I can be sure if i see it.

I've repaired my Joven with parts from another water heater so your one might be the same..

Edit* I'm not suggesting you to fix it yourself as there are live contacts at the top heater element ..
        just making sure that yours is worth the time to ask a technician to repair it..