I just completed the restoration of an old road bike. The only problem that I can detect is the sound of the chain as it goes through the cogs. Instead of the smooth "hrrr" that can be heard in good adjusted bicycles, in mine the chain makes a strong "grrrr". That means that is the same sound as usual but heavier, like the chain doesnt glide nicely on the cogs but instead needs more power on the pedal to move (it doesn't, but it feels that way).
The front derailleur is ok, doesnt intervene or enything, so my guess is that the problem is located in the rear derailleur.
Even when I'm pedaling something doesn't seem right, I don't feel the effortless pedaling I feel in new bikes but rather a not-smooth uncomfortable feeling on my legs.
I hope that made you understand the problem. If not I will upload a video.