  The magnetic field noise in superconducting quantum interference devices
(SQUIDs) used for biomagnetic research such as magnetoencephalography or
ultra-low-field nuclear magnetic resonance is usually limited by instrumental
dewar noise. We constructed a wideband, ultra-low noise system with a 45 mm
diameter superconducting pick-up coil inductively coupled to a current sensor
SQUID. Thermal noise in the liquid helium dewar is minimized by using
aluminized polyester fabric as superinsulation and aluminum oxide strips as
heat shields, respectively. With a magnetometer pick-up coil in the center of
the Berlin magneti- cally shielded room 2 (BMSR2) a noise level of around 150
aT Hz$^{-1/2}$ is achieved in the white noise regime between about 20 kHz and
the system bandwidth of about 2.5 MHz. At lower frequencies, the resolution is
limited by magnetic field noise arising from the walls of the shielded room.
Modeling the BMSR2 as a closed cube with continuous \mu-metal walls we can
quantitatively reproduce its measured field noise.
