2000 Jaguar XJ8 make a "knocking" noise

2000 Jaguar XJ8. I took my car to get the oil change and the next day, the vehicle started to make a "knocking" noise. I checked the oil level to make sure that everything was being lubricated and the oil level was extremely low. I added approximately two quarts (the vehicle takes approx seven), but the problem still persisted. I called the business that performed the oil change and told them that I thought it to be a rod knock. However, when I took it to another mechanic, they told me that the "knocking" was coming from the valve area; not something that could be associated with a faulty oil change. What do you think then? It just seems too extraordinary to be coincidence.