the issue only occurred when I powered the BT module from the 12v to 5v volt. reg? And none at all when I connected it with a different power supply?
It's always the same thing:
The BT module draws pulsed supply current as its radio sends packets. This current circulates in ground, thus creates a voltage on it, and ground is also used as zero volts reference for audio. Thus you hear each bluetooth packet in the output.
In addition the buck regulator will probably switch modes between full PWM when the radio is using a lot of current, and powersaving/cycle-skipping when it is not, which modulates the switching frequency, and that can leak into the audio too.
On the usual devices using bluetooth, this is fixed by doing a proper layout to direct current where it should go, but this is not possible to do with modules: in this case supply current has to go through the wires, there is no other choice but to make a ground loop.
You could use an amplifier with a balanced input to reject the noise.
Another option is an isolated 12V to 5V converter to break the ground loop.
Another solution is to make the bluetooth module's power supply current constant. This can be done by replacing the buck with a shunt regulator, but there is a simpler option. First, replace the buck with a linear regulator (LDO) to get rid of the switching noise. Then put a RC filter at the input of the LDO, with a low cutoff frequency. Say 470-1000µF cap, and the resistor should be calculated to drop 3-4V at the average current drawn by the bluetooth module. Then the bluetooth module's pulsed supply current will come from the cap and not the power supply.