[safari sufferer]

It goes from bad to worst!!!

The mechanic couldn't find the answer to the problem. He did his best, checked everything, try to check the cat by taking off the last piece of exhaust, checked wires, o2 sensors, etc. It seemed to fix itself??? He removed the check engine light with the code reader. It hasn't come on again, and said to bring van in as soon as light comes on so he can check the codes. Drove it yesterday without any problems, although a little weak on power and the temp gauge didn't rise much above 70-80 degrees? Could be a sticky thermostat, about the only thing I haven't changed.
Got up this morning and there is a noticable ticking or knocking noise coming from the engine??!!! What now!!!! Sounds expensive!!
Any thoughts or comments?
Is it safe to drive to the mechanics?
Thanks again.