You should install a S.M.A.R.T. tool (e.g., CrystalDiskInfo or Passmark disk checkup) to know exactly which attribute caused the warning to appear and how far the value is above the threshold.
The most common trigger is an elevated reallocated sector count:
Count of reallocated sectors. When the hard drive finds a read/write/verification error, it marks that sector as "reallocated" and transfers data to a special reserved area (spare area). This process is also known as remapping, and reallocated sectors are called "remaps". The raw value normally represents a count of the bad sectors that have been found and remapped. Thus, the higher the attribute value, the more sectors the drive has had to reallocate. This allows a drive with bad sectors to continue operation; however, a drive which has had any reallocations at all is significantly more likely to fail in the near future.
This means that some sectors of your HDD have already failed! Those might be the last sectors you ever see failing, but it certainly increases the probabilities of a massive disk failure.
Failing sectors certainly don't cause any audible symptoms. Chkdsk only checks the file system, not the disk itself. If the sectors have been successfully, chkdsk won't find any problems.
