I have a single speed bicycle that is less than one year old and probably has a mileage less than 100 km / 60 mi. I've recently noticed that the back wheel on the bicycle is giving out a a squeak on almost every revolution. The sound is similar to the sound of the wheel rubbing against the brake pads, but i've verified that this is not the origin of the sound. What i've also observed is that the squeaking happens only when i'm putting my full weight on the bike seat. It must also be noted that i'm a fairly heavy person i.e. 105 kg / 235 lbs and the maximal rating for the bicycle given by the manufacturer is 120 kg / 265 lbs. What could be the root cause for this issue?
As mentioned in the comments above there could be many causes of the noise as some have listed there. Both @Hursey and @Burki I feel have good theories for the squeeking wheel of your bike. Especially considering the info given in your question, specifically that your riding weight is approaching the limit of the bike, which has rim brakes and is in it's break-in period based on the low milage.
Properly set up rim brakes have 1-2mm of clearance between the rim braking surface and the pad. When off the bike and checking for brake rub by spinning the wheel, one may not detect any rubbing and also find the pad to rim clearance appears adequate throughout the rotation. Note the presence of any run out of the rim as it spins (is there an area of "wobble") even if very small and doesn't contact the pad.
When the bike and wheel are subjected to riding forces (your weight and the force of peddling), some degree of deformation of the frame and rim occur. An already narrow clearance of a mm or two can be lost. A slight run out in an area of the rim can be exacerbated by the weight and force of riding and will now cause rim to pad contact.
Spoked wheels go through cycles of loading and unloading with each wheel rotation. At any point in the rotation, there are spokes straining and bearing the weight and forces of riding while the opposite spokes become slack. This cyclic loading can cause loosening of the spoke nipples, causing the spokes to become unequally tensioned and the rim eventually comes out of true.
Most bikes through a wide range of quality and cost are specced with machine built wheels. Equal and adequate spoke tension (which makes a strong wheel) may not be a trait of most mass produced wheels. With cyclical loading loose spokes become worse quickly. One of the things a quality bike shop will offer with their free post-break in tune up is rim trying or at least checking for adequate and even spoke tension in the wheels.
All this to say that theres a high probability the squeaking is brake rub from rim deformation. You should have a qualified mechanic bring the spokes up to the proper tension.
You may want to check and modify the tension of the seat clamp bolts and also the seat post clamp. These are two spots that can cause noise that can seem to come from elsewhere on the bike. See if the noise persists when you're off the seat.