I was studying Acoustic Resonance and Resonant Frequencies and learned a lot ... but I have a question which probably the web doesn't seem to answer! Can an object(any object)keep changing its resonant frequency: like is the resonant frequency just constant or it can change in the same object
The resonant frequency of an object depends on its internal stress distribution.
For example, when you tune a guitar, you change the resonant frequency of the strings by changing their tension.
The resonant frequency also depends on Young's Modulus, which changes with temperature for most materials.
The resonant frequencies are determined by an object's geometry, material composition, and the boundary conditions placed on the object. Unless the object is self aware it cannot do anything to itself. However, if some of the above properties change in time then, YES, the resonant frequencies will change too. This is an observed phenomenon in musical instruments when you take into account the real motion of all parts of the instrument, the environment, and how the musician attacks the instrument.
There are examples of using valves etc to change the not of a trumpet but I'd say that isn't what you are asking about. If you overdrive a guitar string (play with too much force) you will cause a change in tension high enough to observe a wobble in the tuned frequency. As you play an instrument it will heat up from your body heat and the introduction of energy. Over time this will cause slight deviations in the object's resonant properties. Basically, take the equation for the fundamental frequency as a function of tension, stiffness, air bulk modulus (which depends on temperature), and length (which also depends on length) and ask yourself how any of these things could change in time. That would be your answer.
I had a physics professor with perfect pitch. He once stated that he could hear the change in intonation of a pipe organ during church service, and in different seasons with extreme weather changes.