Thermal effects can generate spurious voltages. Nodes are most sensitive to noise currents if they have a high impedance and carry low level or very accurate signals. An electric field may induce the problem current, or it may come from circuitry connected directly to the node. Because trace impedance rises with frequency, this problem is worst at high frequencies. To disturb a low impedance point, a noise source would have to inject a large amount of current.
Loops are sensitive to magnetic fields if they have a low impedance and a large area.
Any loop that current can flow around may be affected, even if it is made up of several nodes. Most power supplies are slightly affected. Ground, however, has a lower impedance than the complete supply loop and is often very sensitive to noise. Other factors which make a node sensitive include the length and layout of its traces. Traces which pass near a noise source will be affected by that source.