Creaking / cracking during ascent phase - symptom of metal fatigue?
I was on a flight from Muscat to Salalah (Oman) today and noticed a rather disturbing phenomenon. During the ascent, and only during the ascent, the hull (particularly over our heads and towards the left side of the plane) was making quite a loud creaking / cracking sound, like one would expect from a submarine. I've been flying for years and have never encountered a sound like this. Even my wife, who is by no means a nervous flier (which is more than I can say for myself), was noticeably shaken by the sound.
Because the sound only occurred during the ascent phase, and gradually tapered out as we reached cruise altitude, I suspected it may have been related to pressurization. The sound occurred during segments of the flight where there was not even the slightest hint of turbulence.
As a frequent flier I've read a reasonable amount on the subject, and understand the concept of metal fatigue and its relationship to the pressure changes associated with take off / landing cycles. This particular plane (Boeing 737-800) seemed to be at least 10 years old, and as it is being used on a short flight (70 minutes), I'd guess that it has experienced a higher number of cycles than average.
Could this creaking have been a symptom of metal fatigue? Should it be reported to the airline as a possibly urgent maintenance issue?
Thank you in advance for your time!