well im not so sure it is a lifter now. I have cylinders 1,3,5,7, misfiring and the tapping noise is coming from the passenger side. I am almost thinking it could be a knock sensor- doesnt one cause the odd cylinders to misfire and one cause the even. If it is not in sync with the crank pos sensor? I have a O2 sensor running lean on that side too.
thought it could be a coil to but since one runs a combo of even and odd cyl rulled that out. Its just wierd how its only the odd cylinders misfiring.

 

I have never heard of any knock sensor causing half your engine cylinders to mis-fire..but I am not familiarized with the workings of the rover knock sensor. I think it's time to start doing some investigating...fuel, spark, compression test, cylinder leak down test.

 

I had similar symptoms at one time. replaced a cracked exhaust manifold and a bad MAF. This took care of the problem. So you might want to check them both.