help with humming noise?

Try grounding the CA7. You may have a grounding problem that goes away when you hook up a Nak to an ML component that is itself properly grounded. For example, the cable ground/drain on the MLs may be connected to chassis ground. Connecting the CA7 with the PA7 may not provide the same grounding solution. Run a wire from any chassis screw on the CA7 to a known good ground (start by trying any screw on a grounded ML component). If there's a little spark when you complete the connection, you may have a faulty electrical component in the Nak circuitry. This would require a professional service center to fix. If there's no little spark and the hum goes away, you're done.

Yes,there are many threads on hums.

There are also a handful of reasons for hum in your setup with a number of valid and invalid resolutions.

I'm not familiar with either component, but you might have a ground loop between them. Try floating the ground (using a cheater plug) on one of those components and see if that stops the hum. You might also want to mess with placement...just in case it's electrical interference between components. There are a million hum threads archived here, so you should do a search and see if you find your answer.