It is likely to be what is called as Spermatocele
Thanks for your query,based on the facts that you have posted it appears that you have
a marble sized lump behind Rt.Testis.
Most likely this is what is called Spermatocele.(Spermatic Cyst
It is painless ,freely mobile .benign lump that develops due to blockage of the Vas Deferens resulting in to accumulation of spermatic fluid .
The diagnosis is confirmed only with clinical examination by a qualified General Surgeon
scanning of the scrotum.
Normally no treatment is required unless it increases in size rapidly or becomes painful.
Hope I have answered your query. If you have any more questions please feel free to ask me I shall be happy to help you .
Thanks and Regards.