During test observations using the X-band (8--10 GHz) receiver for project GBT02A-041 a spectral baseline ripple with a wavelength of 20 MHz was observed. This feature appears in system temperature measurements as well as the antenna temperture for total power position switching data. The problem appears to be upstream of the LO1 and not created by reflections from the telescope structure or related to the optics. It is also not dominated by any structure in the calibration noise tube.