An air carrier Flight Crew opted to return to the departure airport after experiencing and unexplained 'boom' apparently associated with the right engine. No evidence for the cause was immediately obvious after landing.

Narrative

We were climbing through FL250 in light to moderate rime icing and light chop with both cowl and wing anti-ice on. The flight crew heard a boom and noticed that the #2 engine vibration gauge was no longer displaying a number readout or a needle indication. The pilot flying reduced power on the #2 engine and leveled off at FL260. The pilot monitoring notified ATC that the flight was leveling off at FL260 and asked to stay there as well as notifying ATC of a possible emergency situation. The flight crew observed that all other engine indications were normal and no further noise or vibration was heard. The pilot monitoring pulled out the QRH and looked for vibration guidance but found none. The pilot monitoring contacted maintenance control to advise them of the situation. During the conversation the #2 engine vibration gauge began showing an indication. The vibration indication was 3.8 and at the maximum range of the amber arc. Maintenance Control advised the best course of action would be to return to the departure station. The pilot monitoring then advised ATC of our intentions of returning to the departure station. ATC then asked if the crew wanted to declare an emergency. The crew did not declare an emergency because all other engine indications were normal and the engine vibration gauge was slowly returning to normal. ATC issued a new clearance to return to the departure station. The #2 engine vibration gauge over the course of approximately ten minutes returned to a normal operating range. For the remainder of the flight in the #2 engine responded and indicated normally. Once returning to the departure station; the pilot monitoring advised the flight attendant and passengers that we were returning to the departure station as a precautionary measure. Once at the gate the flight crew pulled up the engine exceedence history data which displayed that the #2 engine recorded a vibration of 4.0 for 75 seconds. There was no visible damage on the outside of the aircraft. We speculated that possible causes for the vibration could have been either ingested ice off of the aircraft or a possible compressor stall. With evidence for neither; the flight crew was unable to pinpoint the actual cause. After the incident the flight crew found guidance in the QRH for vibration which was labeled 'N1 Engine Vibration'.