[peterb]

It is interference generated by the laptop internals (anything ifrom the screen inverter - does the noise reduce if the screen blanks? - to other internal inverters, timer chips etc. There is noise being generated by the external PSU, as the noiser reduces when that is disconnected.

One way to tackle it would be with a special audio transformer that would electrically isolate the laptop output from the PA input - but they aren't cheap., and may not cure the problem if it is radiated interference. (One way to determine that is if the lead is unplugged from the laptop, but held near it - but that isn't conclusive on its own).

The PA probably has a balanced input, compputer do not have balanv]ced output. Some form of audio balun might also work (cheaper than a high quality audio transformer. You also need to look at the impedance of the PA input. Generally these are low, but if you are using a high impedance input, that can cause problems.

As I said in an earlier post on this revived thread, these problems can be very difficult to track down and fix.