Trapped modes in the sliding joints in CESR were investigated as a possible source of instabilities. To detect these modes, experiments were performed where pairs of bunches of positrons were run through at various spacings. It was hoped that constructive and destructive interference of wake fields would produce varying amounts of heating. Data taking was time consuming, and insufficient numbers of data points could be taken to fit to the wake fields. To help solve this, the frequency of the mode was measured on the bench. A single trapped mode and its profile were observed. The trapped mode has a frequency of 3.59 GHz, just below the TM cutoff of the beampipe. Measurements of the mode were used to predict a shunt impedance of 4.9 ohms. The corresponding loss factor is slightly higher than the total loss factor found in previous time domain measurements. The program ZAP was used to predict an instability threshold of about 2.1 A.