yes we had same problem on our plus 4 year 2000 car turned out the rubber filler hose was beginning to fail put in a new one and it fixed the problem see this link for my photos

https://flic.kr/s/aHskkTxRYp

Morgans 1934 MX, 1947 Series 1, 1956 +4 TR4, 2000 +4 T16
Triumph Herald 1969 13/60
Morris 1970