E53 2003 4.4I shacking/rumbling

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I have a 2003 4.4i X5 76k that recently started acting strange.

The best way to describe it is slight rumbling at idle especially when cold. The rpms drop slightly and I feel my seat shakes intermittently. On two occasions, it started and briefly thereafter the rpms went down and the engine died out. I had to depress the accelerator to keep it going. But has only died twice and not since about two weeks ago. Also, I am not sure, but I feel that there is some delay in the pick up while driving -- hard to tell though.

There are no error messages on the OBdII, so I am thinking it is not the coils -- I had messages when a coil failed in the past.

I'm thinking of starting with the spark plugs.

Anybody had similar experience? Thanks.