The correlation of the outputs of two dc-SQUIDs connected to the same solid washer was studied. A dedicated electronic system was used in order to operate both SQUIDs at the same time. It was found that the temporal correlation of the SQUID outputs is strongly dependent on the geometry of the SQUID loop. For a solid washer, the flux noise is mainly due to vortices located close to the SQUIDs. However, by surrounding the SQUID loop with a hole patterned in the superconducting solid washer, it is possible to drastically reduce the flux penetration around the loop. The resulting flux noise becomes dominated by vortices located inside the washer far from the SQUIDs. Magneto-optical images were also used to visualize the flux penetration inside the washer around the slit of such SQUIDs. These confirm previous descriptions of vortex penetration deduced from the electrical measurements.