Given that this sounds like a new and fairly sudden oil loss (is it?) AND the noise, I would have someone check the compression on the engine and look for serious problems. I would not have the same shop do it because any shop that wants to change a timing belt because of a rough idle probably shouldn't be trusted.
It is quite possible that you were due for a timing belt - is that why it was changed? Was anything else done, like spark plugs, wires? You know - things likely to produce a rough idle?
In any case, I'd get a second opinion on the condition of the engine. I doubt that the knocking is just about 5-30 rather than 5-20