230V 50Hz to 15V Switching Transformation
Stray E or H fields are called Common Mode , meaning common to both {signal
and ground} or {power and ground} and when floating or high impedance can cause  the unbalanced input  to convert this to a differential voltage. 
This may be conducted or radiated interference. Each can require similar or different solutions.  
In some cases an unregulated DC supply such as you describe injects conducted noise into the output stage which draws from this voltage.


The most common solutions are;


Identify if it is magnetic radiated noise by moving the source.
Identify if is conducted by measuring the AC ripple voltage on the DC supply.
Shunt the common mode field by earthing or ground connection to a nearby ground.
Add ferrite CM and differential chokes to the DC input.  ( Many wall supplies have CM ferrite clamshells molded inline on the DC cable)
shielding of high impedance signals may be necessary in some cases.
A linear low drop out regulator (LDO) offers the lowest noise rejection but low in efficiency. 
SMPS are common and 50mV ripple is common as well but more efficient.


A PC PSU has 12V. THis can be used for external applications from an unused HDD Molex plug.