What is the bird that sounds a little bit like a "speeded up" chiffchaff?

I can hear a bird that reminds me a bit like a chiffchaff but it sounds faster.  It has just 2 notes, one up one down.  This sequence length sounds random.  Sometimes you hear 5 pairs of notes.  Sometimes it can be as many as 10.  Always, there is then a pause of about 1 second, and then the 2 notes sequence repeats.  What could this bird be?  (My guess is that it's a common bird, nothing fancy.)  Thanks a lot.