Are you getting a "beating sound" ? If so one of the strings is out of tune with the others. You get a "beat note."
If this is not what you're talking about than I'm not sure what you're describing. Does it happen if you pick each string individually and wait until that note decays (goes silent) before picking the next?
Also, is it specific to certain chords? For example, on a uke that has a high nut, and thus poor intonation at the first few frets, the G chord will sometimes sound really terrible because the G on the second string should be in unison with the open G on the fourth string but is not and you get a really noticeable beat note.
Tune the open strings so they are in tune. Then, hold a "G" chord shape but pluck just the 2nd and 4th strings at the same time (thumb and one finger on the right hand doing the plucking). If you hear a "pulsing" sound then the notes are out of tune and it is likely that the uke needs to be set up. A high nut, causing these tuning issues, is very common on many factory ukes; especially, but not limited to, the entry level ones.
BTW, strings are numbered from 1 (closest to the floor, and tuned to "A" in standard tuning) to 4 (closest to your face and tuned to "G" in standard tuning).
John
