[undertow]

Most likely the cam synchro as above, for a test or temporary workaround you can unscrew the black cap and tip a teaspoon of engine oil down there.

Back in the day people used to buy them from rock auto in the states, but there are local options now. The very last year or two of the 5.0 explorers had the same part. The Dorman one was dirt cheap but regarded as poor quality with several tales of the gear stripping or falling off and destroying the engine. I haven't keep up recently, but the genuine Motorcraft part would be the safest bet if it's still available.