[murphyv310]

Hello
Not knowing what valve types you have I will explain to my best what you have.
Valve types 6K8, 6K7, 6Q7, ECH35, EF37, EBC33 ETC all have the Control Grid (G1) connected to the top caps touching them with your finger will induce audio pick up at the mains frequency so this is the hum you hear. On the first two valves if you check voltages on the top caps you are introducing stray capacitance from your meter & leads so effectively altering the tuning of the aerial coil and IF depending on the valve.
Some older valves and TV line output valves the anode is connected to the top cap.
Cheers.
Trevor