Update your post to include full system hardware specs and OS information.
PSU noted as a "Corsair RM550x, new".
Disk drive(s), make, model, capacity, how full?
= = = =
4 - 5 months old:
At best it is quite likely to be some odor right at the beginning (brand new, never used) when by-products of construction and manufacturing burn off of electronics during initial use.
Usually that odor does not last very long and not for 4 - 5 months. Otherwise, the materials may be substandard and the odorous fumes potentially dangerous. Could be design and assembly related as well.
So my thought is that something got unexpectedly hotter than usual, for reasons unknown, and went beyond even the last possible "burn off" odors.
One reason could be that the PSU is not capable of meeting the system's power demands. 550 Watts may not be enough.
Use the power calculators in the following link to determine the wattage requirements. If a component provides a range of wattage values use the highest provided value.
[Read the entire article just as a matter of general information regarding the critical importance of PSUs.]
A very close inspection is warranted. Double check the power ratings/limits for everything. 120 watts may have been just "a watt too far". To me, turning "right" generally means an increase of some sort.
The possible progression being increased odor, smoke, sparks, flames..... Does not end well.