The names Sauron and Saruman are very similar in writing and pronunciation.
Is it an accident? or having an a forethought reason?
As far as I can tell there's no major etymological connection between them. Obviously Tolkien wrote the languages (Quenya/Sindarin) on which both names are supposedly based but I've seen no evidence that he intended the names to be similar.
It's worth noting that in Tolkien's own reading of LoTR, the names are actually pronounced quite dissimilarly, more like "sow-ore-onnn" and "saaar-oo-munn"
The character, and name, of Saruman first emerged in a plot-outline from 1940 that is given in History of Middle-earth 7:
The wizard Saramond the White [written above at the same time: Saramund the Grey] or Grey Saruman sends out a message that there is important news: Trotter hears that Black Riders are out and moving towards the Shire (for which they are asking).
Aside from one brief hesitation (reverting back to "Saramund" in a subsequent outline) Tolkien appears to have definitively decided on "Saruman" from that point forward.
The only discussion of similarity of the names that I'm aware of is in reference to his (partial) renaming in Ralph Bakshi's Lord of the Rings; for example FlyingMoose.org's critique:
They decided to rename "Saruman" to "Aruman" for the movie; evidently they were concerned that moviegoers would confuse the name "Saruman" with "Sauron"...
And Tolkien Gateway:
From early on in the production, it was decided that "Saruman" and "Sauron" sounded too much alike, and might confuse viewers. On concept art, Saruman is called "Ruman", but prior to recording, this was changed to "Aruman". However, during recording, it was again changed, to "Saruman". Because of this late change, several instances of "Aruman" remain in the finished film.
Regarding the names themselves, they are presented as being in two different languages in Tolkien's world.
"Sauron" is a Quenya name; in Sindarin he is called "Gorthaur".
"Saruman" is a Northern Mannish name (actually Old English), presented as a translation of Elvish "Curunír", as the Istari essay in Unfinished Tales confirms:
Now the White Messenger in later days became known Elves as Curunír, the Man of Craft, in the tongue of Northern Men Saruman.
There is no evidence of any author's comments regarding similarity of this name to Sauron's.