Sounds like it may be valve clearence. Do you know if your car uses hydraulic lifters or solid lifters with adjustable rockers. I am thinking hydraulic so maybe a lifter not working correctly. There are some oil additives that can help flush lifters that could be worth a try.

 

Hi mate,
Thanks for the reply. It went to the mechanic this afternoon and they are going to take the rocker covers off tomorrow to investigate. They are saying it could be potentially a damaged Cam and Lifters, but I guess we will know more tomorrow 😪

 

Ok that seems like a worst case scenario for the noise you described so lets hope its just a loose or worn rocker or a noisy lifter.
I had a bit of a search and it seems that some of the HO 351C engines in your year had mechanical cams. If thats the case with yours then its likely just valve clearance adjustment and that should sort it.
Hopefully its just a simple adjustment for you.