Problem : Background noise/high frequency pickup

Sources of noise in the circuit are the PIC board and keyboard ribbon.
To reduce pickup of noise :
Make sure that PIC board is a separate board from VCF/VCA board as described in my layouts - keep the boards as far apart as possible.
Keep keyboard ribbon as short as possible.
Use shielded cable especially from waveshaper to VCF/VCA.
Ground all the shields
Keep TRIG wire away from VCF/VCA board.

Problem : my VCA thumps at end of decay

You need to adjust 1k trimmer to get rid of thump at end of decay. If the thump cannot be removed by adjusting trimmer the following might be wrong :

The back to back transistors are not a close enough match - use a closer matched pair. Many multimeters have a Hfe function for measuring transistors gain to match them.
The power supply is under 11 volts - the supply needs to be at least 11 volts for the VCA to work correctly.
The power supply has large amounts of ripple. One of my old power supplies had more ripple - it caused the VCA thumping problem. Use a nice clean 12 v supply. If you suspect this you can confirm by running from batteries to see if the problem goes away.

Problem : Synthacon filter picks up 100Hz signal at low decay settings OR another ADSR circuit doesn't work correctly.

This problem only shows up with other filters sensitive to control voltage fluctuations like Synthacon Filter (or if you have built another ADSR circuit) :
The TRIG output of the PIC goes high while a key is pressed. Every 10 ms it briefly returns to 0 as the PIC cycles around the keyboard scan again, realises the key is still pressed and makes high again. These 0 going transitions do not have any noticable effect on the circuit as presented. In my synthacon filter the 0 going glitches produce a 100hz signal hum at low decay settings. Also if you used another ADSR circuit that expects a constant high Gate voltage the TRIG output will not work as a gate. This problem has been fixed in latest PICs shipping as at 17 May 2009

For older versions of the PIC the following circuit "fixes" the TRIG signal and makes it a constant GATE signal :