Laptop supplies give off significant AC component from capacitive coupling with the mains, so unless
properly grounded a laptop will tend to float at perhaps a few dozen volts AC, which may be interfering
with your measurements. If your whole circuit is screened to its own ground, this wouldn't be a problem,
but you are no doubt capcitively coupling the signals you are measuring to a true ground, hence inducing
noise.
When you touch the laptop, you are effectively grounding it. [Or perhaps it is grounding you and you were
the source of noise in the first place]