An onload tap changer (OLTC) in suitable conditions is essential for the operation of a power transformer. It is extremely desirable to have indicators that help assess the conditions of the OLTC, especially if these indicators can be used in an on-line monitoring system that does not affect the normal operation of the transformer. This paper describes a method for detecting faults in the tap selector by means of vibration measurements during tap changer operation, using envelope analysis based on Hilbert transform and wavelet decomposition. Different failures at the tap selector can be distinguished in the vibration signature as they are reflected in different parts of that signature. The diagnosis parameters allowing the most selective failure classification are found.