Strange Clicking Noise From Engine Area?

I have a 1998 Volvo S40 2.0Ltr with 155,000 miles on it and I switched
on my car tonight to a strange repetitive clicking noise coming from
the engine bay it seems to stay the same speed and pitch and is on from
as soon as i turn the car on though i have not driven it for more than
a few minutes since it started as i was worried it was the engine!

My ABS unit was just changed recently and the light is always on as the
garage are waiting for me to take it back to finish the job off but the
clicking was there b4 the ABS light came on when I turned the engine on
though the light comes on very quickly!

However when I finished work and drove home, when I turned the engine
off (but kept the lights and power on) I noticed it was still making
that noise.

Putting my ear next to the ABS unit I am certain it is that which is
making the noise!

Despite my ABS failing on me in the summer it never made any noise
before! what is the clicking coming out of the unit?

Is it the ABS pumps? what do they do? and if it is them what is the
worse that could happen if it is left as it is? I guess the ABS unit
would break and the noise would stop and I would just be without ABS,
which considering this reconditioned unit is under warranty would not
cost me anything to fix!
so that wouldn't be so bad except for from inconvenience!

Or would it be unsafe to drive like that? I know in a accident I may
get my wheels locking but my normal brakes would work wouldnt they?
Please if anyone knows can you fill me in! Thanks!