Acclerator noise on Vauxhall Zafira - any ideas?

[mumsy]

i have a 1999 1.6 petrol vauxhall zafira. it has started to make what i can only describe as a "ticking" noise when i put my foot on the acclerator. it gets louder the more i push down and subsides when i take my foot off. has anyone any idea what might be causing this? many thanks