The most common hiss that is referred to is caused by RF interference.
An important step in setting up a system is to tune the equipment properly to ensure that external RF signals are avoided and hence interference is avoided.
The other hiss that is sometimes noticed is the internal workings of the unit itself. The body back receiver has inherent noise from the components especially when there is a low input signal.
This phenomenon is not unique to Sennheiser but occurs across all manufacturers.
This internal noise is made more apparent when in a very quiet environment (like a recording studio) or when more sensitive ear buds (such as custom in-ear molds) are used. From a practical standpoint once you have audio through the IEM system it will over shadow the noise from the internal components and most people will not even realize the noise is there.