In the Halden Boiling Water Reactor (HBWR), a resonant power oscillation with a period around 0.04 Hz is observed at power levels higher than about 9.5 MWt. Although this resonant oscillation is not so strong as to affect the normal reactor operation, it is significant, from the viewpoint of reactor diagnosis, to reveal the cause of this oscillation as well as to understand its characteristics. Noise analysis based on the autoregressive (AR) modeling technique together with spectral and correlation analyses is performed to investigate the driving source, which indicates that it is attributed to the dynamic interference with the reactor of heat exchange process in two parallel-connected steam transformers. The present study demonstrates the effectiveness of the technique applied here for determining the so-called noise source inducing variations of quantities in a system together with its applicability to various problems in the field of reactor noise analysis and diagnosis.